Good fun from Falcon this morning!
A very enjoyable challenge. Not too easy, not too hard. Many thanks to Falcon!

ACROSS
1. Game’s square between yellows and greens in London? (2,2,4)
GO TO JAIL
Cryptic definition
The game in question being Monopoly
5. Paintings in health club in ancient city (6)
SPARTA
ART (paintings) in SPA (health club)
10. Fruit and a soft Italian cheese? No thank you (7)
APRICOT
A + P (soft) + RICOT[ta] (Italian cheese, no TA (thank you))
11. Said about loud noise first, maybe (7)
ORDINAL
ORAL (said) about DIN (loud noise)
12. See head of gorgeous girl in mirror (5)
GLASS
G (head of gorgeous) + LASS (girl)
13. 3’s wife could, struggling alone with a net (3,2,4)
EAT NO LEAN
(ALONE with A NET)* (*struggling)
See 3d:
Jack Sprat could eat no fat,
His wife could eat no lean.
And so between them both, you see,
They licked the platter clean.
14. Stand around fine Barbarian leaving game? (8,4)
TREASURE HUNT
TREAT (stand) around (SURE (fine) + HUN (Barbarian))
18. Charged ahead of varsity rower, perhaps in a vivid colour (8,4)
ELECTRIC BLUE
ELECTRIC (charged) + BLUE (varsity rower, perhaps)
21. Civic leader confused old army men (4,5)
LORD MAYOR
(OLD ARMY)* (*confused) + OR (men)
23. Fabric wife put in drawer (5)
TWILL
W (wife) put in TILL (drawer)
24. Breathe badly? Medicinal drink’s required (4,3)
HERB TEA
25. Judges arrives carrying grip (7)
ARBITER
ARR (arrives) carrying BITE (grip)
* not sure why the clue is plural here!
26. Lucky charm found in the old lady’s bed (6)
MASCOT
MA’S (the old lady’s) + COT (bed)
27. Sticking with her through a depression (8)
ADHERENT
HER through A + DENT (depression)
DOWN
1. Grand, large stove in farmhouse (6)
GRANGE
G (grand) + RANGE (large stove)
2. Cutting a new cloth (6)
TARTAN
TART (cutting) + A + N (new)
3. Salt on fish for nursery rhyme character (4,5)
JACK SPRAT
JACK (salt) + SPRAT (fish)
4. With child, tribe inside creating an obstruction (2,3,6,3)
IN THE FAMILY WAY
FAMILY (tribe) inside IN THE WAY (creating an obstruction)
6. Boy skipped round houses (5)
PEDRO
[skip]PED RO[und] (houses)
7. Second tout in court (6-2)
RUNNER-UP
RUNNER (tout) + UP (in court)
8. That glen drawn extensively (2,6)
AT LENGTH
9. Former PM, Brown, to touch down in a county bordering Scotland (14)
NORTHUMBERLAND
NORTH (former PM) + UMBER (brown) + LAND (to touch down)
15. Even-tempered going round Italian fair (9)
EQUITABLE
EQUABLE (even-tempered) going round IT (Italian)
16. Leash may, unfortunately, for dog (8)
SEALYHAM
(LEASH MAY)* (*unfortunately)
17. Carry weapons to support members (4,4)
BEAR ARMS
BEAR (support) + ARMS (members)
19. Small or large, let it free (6)
LITTLE
L (large) + (LET IT)* (*free)
20. County town originally producing wine (6)
CLARET
CLARE (county) + T[own] (originally)
22. Saw bishop out of bed, having gone up (5)
MOTTO
[b]OTTOM< (bed, B (bishop) out, <having gone up)
Quite enjoyable, thank you, Falcon. APRICOT was my pick. Plenty of knowledge of things British required — I think I counted five clues that needed a bit of research. (That’s an observation, not a complaint!)
Thanks Oriel.
Thanks Falcon, that was pleasant. My top picks were APRICOT, ORDINAL, and my LOI, PEDRO. I couldn’t parse RUNNER UP. Thanks Oriel for the blog.
Also couldn’t understand why 25 was plural.