No idea about the identity of the setter, but I’ve definitely remarked before about puzzles with an unusual preponderance of quirky question marks, and with no fewer than TEN of them in this puzzle, plus two exclamation points, perhaps the one that I may henceforwards refer to as “the Riddler” has struck again. With the benefit of hindsight I don’t think anything here was too tricky, but some unaccustomed slanginess, combined with the aforementioned out-of-the-box cluing and a slightly unusual grid may have acted as speed bumps and, gosh, I just checked and even the mighty Magoo has posted a surely vanishingly rare >10m time on this one. I take it all back, must have been a super-tough and totally Friday-worthy puzzle!
I have two COD candidates to nominate: I really loved the cryptic definition at 2dn, but I also loved the economical construction of 16dn so that comes as a close runner up for me. Lush and abundant thanks to the setter! Before I open up the floor, would anyone be interested in (a) doing a more regular Sloggers & Betters style meetup in London going forwards, perhaps about once a month? (I enjoy the social meetups so much when they happen) and/or (b) a NaCroSetMo in November? For those who haven’t heard of NaNoWriMo, it’s the “National Novel Writing Month”, where every November the idea is to write a 50,000 word novel over the course of the month, freeing yourself from the psychological shackles of excessive perfectionism and quality control. Trying to set a whole 15×15 crossword every day for a month would be totally in the spirit of the thing but perhaps a bit silly, but I don’t know, 3 clues a day, 3 grids in the month, something like that? Or is this a really terrible idea?
Across
1 Winding pass runs across a sacred mountain (9)
PARNASSUS – (PASS RUNS*) [“winding”] “across” A.
The mountain overlooking Delphi, sacred to Dionysus and Apollo and home of the Muses, as any classicist kno.
9 Volunteers piebald horse: hack? (3,4)
TAP INTO – T.A. [Territorial Army = volunteers] + PINTO [piebald horse]
10 British philosopher with crook and bad actor (7)
BENTHAM – BENT [with crook] + HAM [bad actor]. Jeremy Bentham the utilitarian and ongoing resident of a cupboard in UCL.Though “crook” is only really an adjective in benighted Antipodean parts I think? So perhaps the “crook and bad actor” is one and the same individual, a BENT HAM… Thanks 
jackkt for pointing out what was in plain sight…
11 Peak number that is receding (5)
EIGER – REG I.E. [number (plate) | that is], all reversed [“receding”]
12 Hotly pursuing credit — nullify moves to claim that (2,4,3)
IN FULL CRY – CR [credit] “claimed” by (NULLIFY*) [“moves”]
13 Red flower’s transfixed me, that’s what it’s done! (7)
BLOOMED – BLOOD [red flower (as in thing that flows)] “transfixing” ME; whereas a flower as in flora may have BLOOMED.
15 Given a choice, some would go for hot snack (5)
NACHO – {give}N A CHO{ice}, once some of it has gone
17 Given numbers to call by day (5)
PAGED – PAGE [call] by D [day]. To PAGE a book you give it numbers.
18 Yank almost to the end in possession of fine wit (5)
JOKER – JER{k} [yank “almost to the end”] “in possession of” OK [fine]
19 Genuine broadcasting contract (5)
PUKKA – homophone of PUCKER [“broadcasting” contract]
20 Old craft returned carrying a letter from Greek port (7)
KARACHI – ARK reversed [old craft “returned”] carrying A; + CHI [letter from Greek]
23 Insurance arrangement that could be making of Lord? (3,3,3)
NEW FOR OLD – reverse cryptic clue – (FOR OLD*) [“new”] = OF LORD
25 Twelve people — only the last remaining — then this? (2,3)
NO ONE – NOON [twelve] + {peopl}E [“only the last remaining”].
I don’t know about you but this put me in mind of an Agatha Christie plotline – And Then There Were None
27 Grand poser producing plate in kitchen? (7)
GRIDDLE – G RIDDLE [grand | poser]
28 Wrong to cover skin, on reflection, or fail to show a leg? (5,2)
SLEEP IN – SIN [wrong] to cover PEEL reversed [skin, “on reflection”]
29 Departs, determined to track secret service (6,3)
DINNER SET – D [departs] + SET [determined] “to track” (as in follow) INNER [secret]
Down
1 Host forgetting a name gets exposed (6)
PUBLIC – PUBLIC{an} [host, “forgetting” a name = A N]
2 One used to play keyboard with the band? (4,6)
RING FINGER – a cryptic definition that works if you pronounce “used” a different way than your brain wants to. You use your fingers to play keyboard, and the RING FINGER is the one with the (wedding) band on it.
3 Having a fresh look, holds ban to be fair (3,5)
ASH BLOND – (HOLDS BAN*) [“having a fresh look”]
4 Problem with air conditioning one can get to the root of? (5)
SUMAC – SUM [problem] + A/C [air conditioning]. A sumac is a tree and thus you could in theory “get to its roots”.
5 Fabulous, being blessed with unknown amount in gold reserve (9)
STORYBOOK – ST [= saint, = (a) being (who is) blessed] with Y [unknown amount] in OR BOOK [gold | reserve]
6 Supreme idiot’s regularly forgotten clock (6)
SPEEDO – every other letter of S{u}P{r}E{m}E {i}D{i}O{t} [“regularly forgotten”]
7 Pet snake nibbled our goldfishes’ heads! (4)
SNOG – S{nake} N{ibbled} O{ur} G{oldfishes}’ “heads”.
That’s some pretty heavy petting…
8 One taking car in addition, needing transport first (8)
JOYRIDER – RIDER [addition], needing JOY [transport] first
14 Change nothing or do nothing for the better? (4,2,4)
MAKE NO ODDS – a bookmaker makes odds for a betting customer; or in this case doesn’t.
16 Roughly restrain labourer begging (3,2,4)
CAP IN HAND – CA [= circa = roughly] + PIN HAND [restrain | labourer]
17 Lack of space in one’s skip for wreckage (8)
POKINESS – (ONE’S SKIP*) [“for wreckage”, i.e. to be wrecked]
18 Old rebel sailor in speech having zero effect (8)
JACOBITE – homophone of JACK [sailor “in speech”] + 0 BITE [zero | effect]
21 Stack that’s firm about to get knocked down? (6)
CHEAPO – HEAP [stack] that has CO [= company = firm] about it. CHEAPO items may come at a knockdown price…
22 Large filling that is most unlikely to work? (6)
IDLEST – L [large] “filling” ID EST [that is]
24 Other half of sportsman’s leg that’s pulled over line (5)
WAGON – WAG [= (singular of) wives and girlfriends = other half of sportsman] + ON [(cricketing) leg]. A train wagon may be pulled by an engine over a (railway) line.
26 Fence: lightweight, for one’s removed top (4)
OXER – {b}OXER. “Lightweight” is a boxing category; take the top off a BOXER to reveal a type of fence in horse jumping.
This is all getting a bit sporty for me, I think I might need to go have a lie down…
I came to this late, having spent a tiresome evening with a non-functioning left mouse button. As it went quite suddenly, it took me quite a while to discover that that was the problem as it seemed that my laptop had simply frozen – and then rather longer to get back into the swing (or rather the laborious plod) of using just a keyboard. I’d given up any thought of tackling the crossword, but after being away from my laptop for a while, returned to it and suddenly found the offending button had sprung into life again!
But by that time I was feeling even more tired than usual, and struggled through, feeling more and more exhausted. But then it seems that almost everyone else did as well. At least I had no problem with OXER (which I must have come across many times in crosswords, and which went straight in today without the need for checked letters) or BENTHAM (a former member of my old college).
I convinced myself that “Willham” (w/ill/bad actor) was a philosopher, and dithered over MAKE NO ODDS since I completely failed to parse it. I even hesitated over EIGER, not seeing how “reg” was a number (thanks, Verlaine – d’oh!).
And, to cap it all, it strikes me that today is Saturday, which means I’m still a day behind. I’m thinking of cancelling next week altogether and spending the time working on my comprehensive survey of distilled drinkables.