A witty challenge from Steerpike – very enjoyable and well worth the perseverance.
There were a few clues that kept me on my toes, and thankfully the solutions fell into place with “aha!” moments. Great fun, thank you Steerpike.

ACROSS
1. Modification in style (10)
REFINEMENT
6. Discovered vast island by a major landmass (4)
ASIA
[v]AS[t] (discovered) + I (island) + A
9. Monk briefly bared all in bawdy-house (7)
BROTHEL
BROTHER[r] (monk, briefly) + [a]L[l] (bared)
10. Unwillingly leave board of women’s group with sulky look, say (4,3)
WIPE OUT
WI (board of women’s group) + “POUT” (sulky look, “say”)
12. Dead weightlifter? (10)
PALLBEARER
13, 23. Rugby fanatic defending O’Mahony’s initial lapse (3,3)
RUN OUT
RU (rugby, i.e Rugby Union) + NUT (fan) defending O[‘Mahoney] (initial)
15. Poem by idiot about port (6)
ODESSA
ODE (poem) by ASS< (idiot, <about)
16. Sailing equipment moved below in middle of hailstorm (8)
BOWLINES
(BELOW IN)* (*moved) + [hail]S[torm] (middle of)
18. Advice from soldier about posh knees- up (8)
GUIDANCE
GI (soldier) about U (posh) + DANCE (knees-up)
20. Trim Christmas tree (6)
SPRUCE
24. Tyrannical lunatic is perverse (10)
REPRESSIVE
(IS PERVERSE)* (*lunatic)
26. Sweet, virtuous type meets obnoxious Liberal (7)
STRUDEL
ST (virtuous type) meets RUDE (obnoxious) + L (liberal)
27. Obsequious European is given heroin (7)
SLAVISH
SLAV (European) + IS + H (heroin)
28. Brought back some delicious Mayan root crops (4)
YAMS
[deliciou]S MAY[an]< (some, <brought back)
29. Put locks on for keeps! (10)
FORTRESSES
Put TRESSES (locks) on FOR
DOWN
1. Rip off Earl’s gown (4)
ROBE
2. Images of enemy holding old gun up (7)
FOOTAGE
FOE (enemy) holding O (old) + GAT< (gun, <up)
3. Brando horsed around, pinching Duvall’s bottom? Anything goes! (2-5- 6)
NO-HOLDS-BARRED
(BRANDO HORSED) *(*around) pinching [Duval]L (bottom)
4. Placid sheep picked up fungal infection (6)
MILDEW
MILD (placid) + EW (sheep, “picked up”, i.e. sounds like EWE)
5. No worse off with Mike in press office (8)
NEWSROOM
(NO WORSE)* (*off) with M (Mike)
7. Dangling purse from bridge crossing river in Oregon (7)
SPORRAN
SPAN (bridge) crossing R (river) in OR (Oregon)
8. Amazed a heavyweight is stuck in outhouse (10)
ASTONISHED
A + TON (heavyweight) + IS stuck in SHED (outhouse)
11. Pretentious prose plagues papers, peculiarly (6,7)
PURPLE PASSAGE
(PLAGUES PAPERS)* (*peculiarly)
14. Sumptuously feast with rogue, oddly lacking cunning (10)
GORGEOUSLY
GORGE (feast) with [r]O[g]U[e] (oddly lacking) + SLY (cunning)
17. A top university hydrologist essentially takes care of bay (8)
ACAPULCO
A + CAP (top) + U (university) + [hydro]L[ogist] (essentially) takes CO (care of)
19. Temporary trainee almost gets one million (7)
INTERIM
INTER[n] (trainee, almost) gets I M (1 million)
21. Exposes international organisation, ruining lives (7)
UNVEILS
UN (international organisation) + (LIVES)* (*ruining)
22. Teachers vacated indifferent lodgings (6)
BEDSIT
BEDS (teachers, i.e. B Eds) + I[ndiferen]T (vacated)
25. So enthralled by enthusiast (4)
THUS
[en]THUS[iast] (enthralled by)
In 10a where does the e come from?
Thanks
Just what I was thinking, Oldham.
Oldham @1. The ‘say’ in the clue denote a homophone. So it is WI followed by a homophone of ‘pout’. PS Still don’t like setters using ODESSA rather than ODESA but that’s probably just me.
The definition of 10ac is ‘Unwillingly leave board’ and the reference is to surfing I believe. My solving experience mirrored that of Teacow’s.
There was much to like in this grid; the ‘dangling purse’, ‘keeps’ and ‘unwilling to leave board’ (if I could just account for the missing ‘e’).
Thanks to Steerpike and to Teacow for parsing a few which eluded me.
Agree with Steven on the definition of 10a.
Hovis, I’m being dumb, maybe, but I don’t get it. A homophone of what?
Thanks for clearing up the def for 10a, Steven @4. I was struggling to see how ‘Unwillingly leave’ could be the same thing as WIPE OUT, but the addition of ‘board’ to the def makes perfect sense of it.
A pleasant Monday puzzle that wasn’t too difficult though I found ACAPULCO wasn’t the first ‘bay’ to come to mind and GORGEOUSLY and GUIDANCE held out till the end. Favourite was the ‘Dangling purse’ for SPORRAN.
As a general observation, yesterday’s NUT seems to have become today’s BEDS in crossword land.
Thanks to Teacow and Steerpike
Diane @6. I’m just saying that the PE OUT bit in WIPE OUT is a homophone of POUT. So WI + “POUT”.
OK, Hovis, thanks.
My last one in was 11D: I know the expression “purple prose” but have never encountered the solution to this clue. Otherwise it was pretty easy.
10A – what a great instrumental by The Safaris!
Thanks Steerpike and Teacow
I thought it unfortunate in 14 that -OUSLY appeared in both clue and solution.
11ac: brilliant. Alteration in both the clue and the solution. Very clever.
Alliteration.
Auto correct!
Late to the party – we solved the puzzle early but didn’t get time to comment yesterday. It’s all been said, really; our only (slight) problem was that we didn’t get the surfing reference in WIPE OUT.
Thanks, Steerpike and Teacow.