Across:
1 Puts a team together (FIELDS a team) in department (-D-) swamped by mixed-up (= anagram indicator) files (files = FIEL-S) (FIELDS),
4 Jelly (AGAR-AGAR) fish (-GAR-) found behind (= position indicator) area (A-) on a (-A-) torn-up (= reverse indicator), low-quality paper (rag = -GAR) (AGAR-AGAR),
9 Oversees (= reverse indicator) accountant (CA = -AC-) in shelter (lair = R-IAL) (RACIAL) – it’s a cultural thing (RACIAL),
10 That’s right (CONFIRMS) for business- (business/company = CO-)-to-new- (-N-) -businesses? (-FIRMS) (CONFIRMS),
12 The role (CHARACTER) of Nature? (CHARACTER),
13 Eddy (eddy/whirlpool = SWIRL) and Ned ignored by swindler (‘Ned’ ignored by ‘swindler’ = swilr = SWIRL),
14 Take a breath (SIGH-) on ridiculous (= anagram indicator) gradient (gradient = -T-READING) (SIGHT-READING) going on the stage without any preparation (SIGHT-READING),
18 Obsession of jockey perhaps (ONE-TRACK MIND) aboard (ON-) last horse (last letter of ‘horsE’ = -E-) at The Curragh? (-TRACK) Keep an eye on him! (MIND) (ONE-TRACK MIND),
21 Take a piece from (UNARM) organisation (United Nations = UN-) with a (-A-) return (= reversal indicator) address (Mr = -RM) (UNARM),
22 One (I-) note (-N-) on the essence of right (-CORRECT) (INCORRECT) and wrong (INCORRECT),
24 Obstruction (BLOCKAGE) case (case = B-AG-) involving rugby player from the scrum (-LOCK-) and second centre (second ‘cEntre’ = -E) (BLOCKAGE),
25 Country (MEXICO) doctor (Medical Officer = M-O) covers for former partner (-EX-) in charge (In Charge = -IC-) (MEXICO),
26 Opposed to (CONTRARY to) criminal (CON-) trial (TR-Y) involving a (-A-) third party (third ‘paRty’ = -R-) (CONTRARY),
27 Those doing a bit of dealing (pushers) lose head (p) (‘pushers’ without ‘p’ = USHERS) with those working in the house (USHERS working in The House/Dáil).
Down:
1 A bet (FORECAST) backing (FOR-) English (-E-) players (-CAST) (FORECAST),
2 Dialogue (EXCHANGE) in first edition (first ‘Edition’ = E-) at times (-X-) from Metamorphosis (-CHANGE) (EXCHANGE),
3 Something for half of those in 1 down (DRAMA for half of ‘forecast’ = cast) producing histrionics (DRAMA),
5 What you might say if someone you don’t like is going (GOOD RIDDANCE) to virtuous (GOOD) religious leader (‘Religious’ leader = R-) for papers (-ID- papers) from Charleston (-DANCE) (GOOD RIDDANCE),
6 Sounds like (= homophone indicator) order to bouncer to not allow the man downstairs (devil = Nick) in (‘refuse nick’ = ‘REFUSENIK’) with dissident (REFUSENIK),
7 One of those involved in underground movement (GERBIL), for example (e.g.), recalls (= reversal indicator) (e.g. = GE-) writing (from the 3R’s = -R-) most of the proposed legislation (most of ‘bill’ = -BIL) (GERBIL),
8 Aftermath (RESULT) of Ulster (Ulster) Question? (= anagram indicator) (Ulster = RESULT),
11 A person of note (STREET SINGER) responsible for the numbers heard on the pavement (STREET SINGER),
15 St Paul’s job (St Paul was a TENTMAKER) as a Galician wine (TENT-) producer (-MAKER)? (TENTMAKER),
16 It’s very similar to the person (LIFELIKE) in biography (LIFE-) and made-up stuff (-LI-E) about Boltzmann Constant (-K-) (LIFELIKE),
17 Issues (EDITIONS) upsetting (= reversal indicator) leaders of North of Ireland tourism (leaders of ‘North Of Ireland Tourism’ = noit = -TION-) divides Ulster, for one (Ulster = side = EDI -S) (EDITIONS),
19 A fair amount of (half) grill (quiz) (half of ‘QUiz’ = QU-) for community (European Community = -E-C) found around Belgium (-BE-) (QUEBEC) city? (QUEBEC),
20 Pool (LAGOON) the French (‘the’ French = LA-) bank’s gold bullion (banks ‘GOld bulliON’ = -GOON)? (LAGOON),
23 Snorkelling spots (REEFS) for lough (Lake REE-) party (Sinn Féin = SF) reviewed (= reverse indicator) (SF = -FS) (REEFS).