A thoroughly enjoyable solve from Goliath.
Some excellent clues at just the right level of tricky and a great range of clue types. Thanks to the setter!

ACROSS
1. Like gold, the bum wobbles and it’s highly visible (1,4,5)
A SORE THUMB
AS (like) + OR (gold) + (THE BUM)* (*wobbles)
6. Part of church starting out in error (4)
APSE
[l]APSE (error, starting out)
9. George Orwell or George Sand? (3,2,5)
NOM DE PLUME
Cryptic definition
Two examples of noms de plume
10. Solipsistic expression of cultural item (4)
MEME
ME ME (solipsistic expression)
12. You’re fiddling with my lists for some strange reason (12)
MYSTERIOUSLY
(YOU’RE with MY LISTS)* (*fiddling)
15. Kitchen feature lacking whisky blowing off troublemaker (9)
HOBGOBLIN
HOB (kitchen feature) + BLO[w]ING* (lacking W (whisky, NATO alphabet), *off)
17. Mike in band break (5)
SMASH
M (Mike, NATO alphabet) in SASH (band)
18. Criminal amongst us coming back, run! (5)
THIEF
Amongst FT< (us, <coming back): HIE (run)
19. Welsh county’s fancy instrument (9)
GLAMORGAN
GLAM (fancy) + ORGAN (instrument)
20. Sounds like their genetic material’s been rejected both ways (5,3,4)
THERE AND BACK
“Sounds like”: THEIR (i.e. THERE) + DNA< (genetic material, <been rejected) (i.e. AND<, <back)
24. Stand for Rupert? (4)
25. Smart denim fashion plan (10)
MASTERMIND
(SMART DENIM)* (*fashion)
26. Team leaves the Spanish flat (4)
EVEN
[el]EVEN (team, leaves EL (‘the’, Spanish))
27. A hundred times returned rogue escaper into the web (10)
CYBERSPACE
C (a hundred) + BY< (times, <returned) + ESCAPER* (*rogue)
DOWN
1. Isaac Newton’s spots (4)
ACNE
2. Some resistance beginning on His Majesty’s Service (4)
OHMS
O[n] H[is] M[ajesty’s] S[ervice] (beginning)
3. Always happy ending and mellow about love? Not always (5,2,5)
EVERY SO OFTEN
EVER (always) + [happ]Y (ending) and (SOFTEN (mellow) about O (love))
4. One composer left among many (5)
HOLST
L (left) among HOST (many)
5. Notes from compiler: I left Shakespearean heroine for lead on Othello (9)
MEMORANDA
ME (compiler) + M[i]RANDA (Shakespearean heroine, I left for O[thello] (lead on))
7. They found navy recruits in accidents involving volatile gases (5,5)
PRESS GANGS
PRANGS (accidents) involving GASES* (*volatile)
8. At last the real McCoy, that’s all (10)
EVERYTHING
[th]E (at last) + VERY THING (real McCoy)
11. Badly roofed, as such a precarious structure (5,2,5)
HOUSE OF CARDS
(ROOFED AS SUCH)* (*badly)
13. Kentish town is a bit steady (10)
WHITSTABLE
WHIT (a bit) + STABLE (steady)
14. Former student taught to destroy (10)
OBLITERATE
OB (former student, old boy) + LITERATE (taught)
16. Fabulous lady with green salad (9)
LEGENDARY
(LADY with GREEN)* (*salad)
21. In combat, hero gets to ablute (5)
BATHE
22. Simone and Juliet leaving warrior (4)
NINA
NIN[j]A (warrior, J (Juliet, NATO alphabet) leaving)
23. He or she supporting commercial cutter (4)
ADZE
ZE (he or she) supporting AD (commercial)
I agree there were plenty of nice clues, and I ticked EVERY SO OFTEN, and HOUSE OF CARDS
All parsed. I do have a question: why is HOLST “one composer”? I know he was a composer, but what is the significance of “one”?
Thanks Goliath and Oriel
Good fun.
Ze was new to me. I guess he or she is a bit cumbersome, but I’ve always got around it with they, despite its being plural to the purists.
The two UK geography clues were a challenge, but I have Pelham to thank for suggesting a week or two ago that I remember GLAMORGAN.
The clue for NOM DE PLUME would be at home in a non-cryptic puzzle.
Martyn, I think the “one” is just to juxtapose the “many”.
Very enjoyable. Liked house of cards, thanks both.
In 7D I think you double counted the A
Thanks for the blog , very good overall and the same ticks as Martyn@1 , but not sure that two clues actually work .
Jay@5 mentions PRESS GANGS but it is not the blog , the clue itself leads to an extra A .
THERE AND BACK – genetic material gives DNA = AND BACK so no need for rejected . Or genetic material’s been rejected simply gives AND . Is there a third way ?
Like Roz#6 I was a bit puzzled by the “…and back”. Like Geoff#2 I had to infer the existence of “ze”.
There are actually two composers called Holst: Gustav and his daughter Imogen.
…Which reminds me of a clue many years ago (in the Independent I think) which was something like: “Two artists produce a third (5)” producing “Johns”. (Augustus + Gwen = Jasper)
Apologies for the two clues highlighted by Roz and others. Sloppy checking on my part
Thank you Goliath , apologise and all forgiven , the rest was great .
Sloppy checking by test-solver and Editor really .
When it gets to the level of “ZE” and ” ME ME”, I am not impressed. “HIE” , for run.
Does “there and back ” really work?
4(d), why the “one”?
7(d) just doesn’t work ?
Some very accomplished clueing, but some stinkers, too.
David wins this one.
Thanks Goliath. I enjoyed this with my top picks being EVEN, MEMORANDA, and HOUSE OF CARDS. I’m not a fan of ‘beginning’ by itself to indicate an acrostic of multiple words as in OHMS but I knew what was meant. I missed the clever MEME and the nho GLAMORGAN. Thanks Oriel for the blog.
Thank you Goliath–minor point in a tremendously enjoyable puzzle.
Excellent. Thoroughly enjoyed this one. I’ve occasionally been a bit critical of Goliath but for me this was near perfect.
Thanks to all.
ZE for “he or she” is one of those attempts at language innovation that never really took off, in part because it feels so forced. Singular “they” is now the solution of choice: to refer to an individual of unknown gender, it has a surprisingly long pedigree; so the much more recent use of referring to someone of known but nonbinary gender is not that far-fetched.
Mike leading to SMASH made me think of DJ Mike Smash; the Harry Enfield creation.
The things I wasn’t sure about have been covered precisely by Roz. Thanks for dropping by Goliath. I also didn’t know ze, but I knew the solution.
A good puzzle. I liked CYBERSPACE, OBLITERATE; HOUSE OF CARDS and A SORE THUMB.
Thanks again Goliath and Oriel