Across:
1 Literary agent drops elegant (‘literary agent’ drops ‘elegant’ = itrary = RARITY) collector’s item (RARITY),
4 Slippery (= anagram indicator) US cobras (US cobras = SCABROUS) can be rough and unattractive (SCABROUS)
9 Starters of pecorino, edam (starters of ‘Pecorino Edam’ = PE-), preserves (cans = -CANS) (PECANS) and nuts (PECANS),
10 Send up (SATIRISE) to satisfy (sate) nearly all (e) (‘sate’ without ‘e’ = SAT-) in upper (‘In’ upper = -I-) stand (-RISE) (SATIRISE),
12 A piece of dance music (BOSSA NOVA) to master (BOSS-) for a (-A) Radio station (Radio NOVA) (BOSSA NOVA),
13 Undergo a sea change (ALTER) at top of the church (altar) state (= homophone indicator) (‘altar’ = ‘ALTER’),
14 Providers of pictures (ILLUSTRATORS) of those making a scene (ILLUSTRATORS),
18 Follows Spring (SUMMER) to college (SCHOOL) (SUMMER SCHOOL) providing space for politicians and commentators to offer views, when teachers are away (SUMMER SCHOOL),
21 A herb (a herb) doctor (= anagram indicator) (a herb = REHAB) produced a course of treatment (REHAB),
22 Live on the edge of (RESIDE-) NYC (NYC) playing (= anagram indicator) (NYC = -NCY) (RESIDENCY) regular gig in the pub (RESIDENCY),
24 Group of shops (CH-AIN) around a (-A-) place (-PL-) (CHAPLAIN) keeping the faith in the units (army CHAPLAIN),
25 Tip from opticians (‘tip’ from ‘opticians’ = ocians = CASINO) – avoid this if you don’t like taking risks (CASINO),
26 She clued (she clued) puzzle (= anagram indicator) (she clued = SCHEDULE) to plan (SCHEDULE),
27 Harry (harry/pester = BADGER) has awful (BAD-) head gash (head ‘Gash’ = G-) and is in hospital (-ER) (BADGER).
Down:
1 Agent (rep) is not finished (p) (‘rep’ without ‘p’ = RE-) with the people (-PUBLIC) (REPUBLIC) on this island (REPUBLIC),
2 Music (ROCK music) on the old boat (sailor = old SALT) (ROCK SALT) synonymous with The Cellar (ROCK SALT / salt cellar),
3 Dance (= anagram indicator) tango (tango = TONGA) in friendly islands (TONGA in ‘Friendly Islands’),
5 Mark (mark/identify = CHARACTERISE) puts his heart (‘hIs’ heart = -I-) into roles (CHARACTER-S-) with energy (-E) (CHARACTERISE),
6 Bikers risk irritating leaders of (‘Bikers Risk Irritating’ leaders of = BRI-) taxi (cab = -C-A-B-) drivers’ motoring organisation (Royal Automobile Club = RAC = -RAC) (BRIC-A-BRAC) with old curiosities (BRIC-A-BRAC),
7 Old (O-) tape (tape) playing (= anagram indicator) (tape = -P-ATE) contains single (-I-) (OPIATE) produced by The Chemical Brothers perhaps (OPIATE),
8 School (S-) gets wind of (-HEARS) (SHEARS) one of those responsible for the cuts (SHEARS),
11 Try in a major way (COURT-MARTIAL) to woo somebody (COURT-) with marital (marital) trouble (= anagram indicator) (marital = MARTIAL) (COURT-MARTIAL),
15 Had words with (SQUABBLED) police force (squad) briefly (d) (‘squad’ without ‘d’ = SQUA-) at opening of bar (at opening of ‘Bar’ = -B-) and ran, in a colourful way (-BLED) (SQUABBLED),
16 Bloodsucker (COUNT-) in (-IN-) Government (-G) (COUNTING) working in the Department of Finance (COUNTING),
17 Borstal boy (Brendan Behan’s Borstal Boy = PLAY-) has a reputation for fighting locally (fighter ‘locally’ = -GOER) (PLAYGOER) with one of those attending the abbey (one of those attending The Abbey Theatre = PLAYGOER),
19 Supports (BRACES) prop (prop/support = BRACE-) for first scrum (first ‘Scrum’ = -S) (BRACES),
20 Heard (= homophone indicator) Moore (‘moore’ = ‘moor’ = -HEATH) joins society (S-) first (= position indicator) (SHEATH) – it’s a cover up (SHEATH),
23 Crisis (DRAMA) witnessed by 17 down (DRAMA witnessed by ‘playgoer’).