My clue of the day is clearly 7dn with its great definition, closely followed by 25ac with its great wordplay! I also liked 10ac, with no disrespect to the alumni of that great institution, and 24ac. Thanks to the setter for a very enjoyable puzzle.
Clues are in blue, with definitions underlined. Answers are in BOLD CAPS, followed by the wordplay. (ABC*) means ‘anagram of ABC’, with the anagram indicator in bold italics. Deletions are in {curly brackets}.
Across
1 One and a hundred is a thousand – that’s wrong (8)
SOLECISM: SOLE (one), C (a hundred), IS, M (thousand).
5 America on bottom row (6)
RUMPUS: RUMP (bottom), US. The definition is “row”, rhyming with “cow”, not “tow”. Ah … English pronunciation: such an aid to misdirection by setters.
9 Single housing worker going begging (8)
UNWANTED: UNWED (single), around ANT (worker).
10 Be cleverer than foolish undergraduate? (6)
OUTWIT: O.U. (Oxford University), TWIT.
12 Lack of craft, perhaps, about a digital processing instrument (4,8)
NAIL CLIPPERS: NIL (lack of), CLIPPERS (craft), all around A.
15 Biblical character I whine about (5)
NAOMI: I MOAN reversed. A golden oldie.
16 Bunch attending one musical — and another? (9)
HAIRSPRAY: HAIR (a musical), SPRAY (bunch).
18 Flower – bottom part raised, we hear? (9)
BUTTERCUP: UP (raised), preceded by BUTTERC (sounds like BUTTOCK … well, more or less sounds like; really, just go with it, people!).
19 Deck, where sailors joining a party (5)
ADORN: RN (sailors), joining A DO (party). So “deck” is a verb, and has nothing to do with ships.
20 Lack of interest concerning argument (12)
INDIFFERENCE: IN (concerning: definition 15 of 17 in Chambers for “in” as a preposition), DIFFERENCE (argument).
24 Marks in Spanish listed when failing (6)
TILDES: (LISTED*). I realised the definition might be an accent, and ran through acutes and graves without luck before finally recognising the anagram!
25 Everything considered, suggesting tall helot? (3,2,3)
ALL IN ALL: ALL (everything) in THE LOT (everything, again). I biffed this confidently looking at the helpers, but then took ages to parse it. Very clever indeed!!
26 Want relief primarily in the end (6)
DEARTH: R (“relief”, primarily) in DEATH.
27 Where to work out if reliability’s back in faulty gas main (8)
GYMNASIA: (Y GAS MAIN*). The Y comes from the back of “reliabilit-Y”.
Down
1 Swig punch (4)
SLUG: a straightforward double definition, where “punch” is a blow, not a party drink.
2 Dirty linen evidently washed, disgusting originally (4)
LEWD: first letters (“originally”) of words 2-5.
3 Singing in Cambridge mansion, roof coming off (9)
CANTABILE: CANTAB (“Cantab” is inter alia the post nominal suffix indicating a degree from the University of Cambridge); then knock the first letter off “PILE” to give an answer I didn’t know although it certainly seemed plausible.
4 Writing tool, magic marker? (12)
SPELLCHECKER: SPELL (magic), CHECKER (marker … of exams, for example, I suppose. A slight stretch perhaps?).
6 Central parts of industry reassuringly, and quietly, appropriate (5)
USURP: US (central parts of ind-US-try), UR (central parts of reass-UR-ingly), P (quietly). “Appropriate” here is a verb not an adjective.
7 There she goes to blunder into catastrophe, captive initially (6,4)
POWDER ROOM: POW (captive) initially, followed by ERR (blunder) inside DOOM (catastrophe). Delightfully disguised definition!
8 Enjoyable day is going well, draining lake (10)
SATISFYING: SAT (the required day), IS, FLYING, without L for lake.
11 See about lesser VIP somehow becoming megastar (5,7)
ELVIS PRESLEY: ELY (the most common diocese in Crosswordland, surely), around (LESSER VIP*). For some reason I found this very difficult to see even after writing down all the letters. Only when I looked at all the helpers and debated where to put the “V” did the answer jump out.
13 A UK citizen holds I require bottles to be drunk (10)
INEBRIATED: “BRIT” holds “A”, then “I NEED” holds all of that.
14 Meaty stuff signalled, a trombone part coming up (10)
MORTADELLA: look for the backwards hidden answer.
17 Post contains rubbish: foolish ultimately opening it (9)
STANCHION: (CONTAINS H*), where the H is the ultimate letter of “foolish”. On edit: to expand on the wordplay, “contains rubbish” calls for an anagram, “foolish ultimately” gives you an “H”, and “opening it” says to put the “h” in the anagram.
21 Speeding boats (5)
FLEET: another double definition.
22 Without allure, nappies emptied (4)
SANS: SA (allure), NS (outside letters of “nappies”).
23 A mischievous thing coming up that may irritate? (4)
FLEA: A, ELF all written from bottom to top.
I guessed CANTABILE but had to use an aid to figure out how the clue was parsed.
I thought ALL IN ALL justified the entrance fee, and SOLECISM and HAIRSPRAY were bonuses. Actually, come to think of it (thanks for the clear reminders, B!) there was a plethora of fine clues, good entertainment for a Saturday morning.
LOI was 7d which I agree is a great clue.
I could not parse everything and will look again at All in All to understand exactly how it works.
Had Shot at 1d for a long time. Was very pleased to finish this. David
However, I’m still trying to work out the parsing for 17 down. Was “opening it” necessary for the clue? Doesn’t it work as well without it? (Also, re: 22 down, was “SA” really ever used to mean sex appeal, or is it just a crossword thing?)
Thanks as always for the brilliant blog, and subsequent comments.