Disclaimer: as usual, this is posted shortly before the solutions are officially revealed at the Crossword Club site, so these suggestions should be regarded, to borrow a term from watchmakers, as water resistant rather than waterproof.
Corrections / additions welcomed, especially regarding 6d.
ACROSS
1 supawn SU(PAW)N – porridge, if you’re an American Indian
4 stickjaw STICK,JAW – seems to have rather a vague meaning. Oxford Shorter gives ‘Toffee; pudding etc. anything difficult to chew’
10 acock-bill A,COCK-B,ILL – a nautical term for ‘one end up’, as ‘Topped up; having one yardarm higher than the other.’
11 pedro – reversed hidden word. The five of trumps in certain card games
12 pitarah (P(I)TARAH)* – ‘I’ inside an anagram of ‘apart’ + h(ot). A travelling box for clothes in India
13 tailzie TAIL,Z,IE – (Scots Law) An entailment or deed whereby the legal course of succession is cut off, and an arbitrary one substituted.
14 carob CA,ROB – a chocolate substitute
15 heptagon HEP TAG ON – you’re pretty cool, man, if you have a hep tag on your gear
18 euonymus (E(U)ONYM)*,US – a bit of a crossword standard
20 jambo JAMB,O – an E African salutation (from Swahili). Immortalised for me, as a connoisseur of cheesy Europop, by German beat combo Mr.President in Coco Jambo
23 aquafer A,QUAF(f)ER – usually ‘aquifer’ but the wordplay seems to demand the alternate spelling
25 tizzies – SEIZ(e) + ZIT all reversed. The capitalization of the literal ‘States is a mean trick.
26 veal – another cunning capitalization, this time of ‘Desert’, the half-way stage literal. It’s DESERT = LEAVE, then swap the ‘old figures’ (Roman L and V) to give you the actual literal – ‘Shield’, here the old form of ‘veil’. Phew!
27 liege-lord LIE,GE-L,O,RD – ‘set’ for ‘gel’ took a long time to click
28 let loose LET LOOS,E
29 hadron HAD,RON – ‘nor’ = ‘and not’ reversed
DOWN
1 stamp act STAM,P ACT – ‘stam’ = ‘mats’ (gets tangled) reversed + ‘pact’
2 proctor PRO,(a)CTOR – ‘backing’ is ‘pro’ (ie. ‘for’)
3 wake-robin W(A,K,EROB)IN – hands up all of you who knew the wake-robin (Trillium erectum). Very clever wordplay, with ‘carried aloft’ indicating a reversal of ‘bore’.
5 teletypesetter Here goes (deep breath): T(h)E,LE(TYPES,E,)TTER
I give you Wikipedia: “A teleprinter (teletypewriter, Teletype or TTY for TeleTYpe/TeleTYpewriter) is a now largely obsolete electromechanical typewriter that can be used to communicate typed messages from point to point and point to multipoint over a variety of communications channels”
6 cippi C,IP,PI – okay, one of my grey areas. Cippi [/ˈsɪpaɪ, ˈsɪpi/] is the plural of CIPPUS – in classical architecture a stele or pillar, so ‘stocks’ in the sense of ‘pillars’. I believe IP is a financial abbreviation but I started wading through various market-related websites and came up with ‘interest point’ and a few others, then I got really bored and went off to watch some paint dry. So sue me. I’m glad I didn’t spend any more time searching financial websites for this – see DorestJimbo’s definitive comment below.
7 judezmo JUD(g)E,Z,MO – aka Ladino, a Romance language of Sephardic Jews, based on Old spanish and written in the Hebrew script.
8 woofed WOO(FE)D – very neat little clue (the metal is iron, FE). I love the definition ‘produced bark’.
9 xiphihumeralis – XIP,HI(HUMERALI)*S – ‘snaps after lifting’ is the reversal of ‘pix’ (ie. pics); then ‘his bags’ indicates ‘his’ as a container for the anagram of ‘mule hair’. Try slipping that one into conversation (unless you’re a cat – it’s a feline bone).
16 al-jazeera AL(JAZ,EER)A – ‘done with’ is ‘à la’, as in ‘duck a l’orange’ – duck done with orange (sounds better in the Anglo-Saxon, you ask me). But this recipe requires a stuffing of unfinished Jazz and a poetic e’er.
17 mouse-dun MO,U,SE-DUN – MO (medical officer) + [a reversal of NUDES + U (‘for all to see’ in British film classifications)]
19 ululant concealed in alternate letters, as – oUt LoUd LeArNs To
21 mainour MAIN,OUR – MAINOUR, crim. law. “The thing stolen found in the hands of the thief who has stolen it; hence when a man is found with property which he has stolen, he is said to be taken with the mainour, that is, it is found in his hands. 2. Formerly there was a distinction made between a larceny, when the thing stolen was found in the hands of the criminal, and when the proof depended upon other circumstances not quite so irrefragable; the former properly was termed pris ove maynovere, or ove mainer, or mainour, as it is generally written.”
22 varvel V,(m)ARVEL – a “metal ring attached to the end of a hawk’s jess and connecting it to the leash”, often engraved with the name or insignia of the hawk’s owner
24 fuero – s(ure of)* – Spanish legal term for a code or charter
At 6D it’s C-IP-PI where C=speed of light=constant and IP is “one penny” being the “minimum price”
I thought this a high standard puzzle that required a lot of concentrated effort to solve. Thank you setter.