Across:
1 Seems a (seems a) mess (= anagram indicator) (seems a = SESAME) or the seed of one? (SESAME seed),
4 In Paris, I (in Paris/French ‘I’ = JE-) get a (-A-) very bad (-LOUSY) (JEALOUSY) feeling of envy (JEALOUSY),
9 Fraud (MISUSE) by mysterious missing Tory (‘mysterious’ missing ‘Tory’ = mseius = MISUSE),
10 What the Customs are responsible for (SEIZURES) in fits (SEIZURES),
12 Dark arts (THE OCCULT) produced by that man (-HE) in bed (cot) in a roundabout way (= reverse indicator) (cot = toc = T- OC-) with religious group (-CULT) (THE OCCULT),
13 Opening bloom (opening ‘Bloom’ = B-) on aster (aster) is cut (r) (‘aster’ without ‘r’ = -ASTE) (BASTE) to keep it moist (BASTE),
14 The change of position for those going to and from church service (MASS MOVEMENT) in March (march/protest = MASS MOVEMENT),
18 Racket (BUSINESS) near playground (PARK) (BUSINESS PARK) in a place where those synonymous with dealing have concerns (BUSINESS PARK),
21 Representative (AGENT) of a (A-) lady’s man (-GENT) (AGENT),
22 Lounge (LIE) popular (IN) with Mark? Time (mark time = WAIT) (LIE IN WAIT) to be patient – it has a surprising effect! (LIE IN WAIT),
24 Softens the blow (CUSHIONS the blow) for those lying about the queen (CUSHIONS lying around the queen bed),
25 Grace (grace/style POLISH) Shine? (POLISH),
26 A fair amount of (half) muck (‘dirt’ = DI-) and top soil (top ‘Soil’ = -S-) – the type you can get from a garden centre (aster sold by garden centre = -ASTER) (DISASTER) leads to reverse in fortune (DISASTER),
27 It’s more spiteful (MEANER) to get out of agreement (‘get’ out of ‘agreement’ = aremen = MEANER).
Down:
1 Banks say (banks 'SaY' = SY-) it's around mix-up (around 'Mix-uP' = -MP-) on Tom's (-TOMS) (SYMPTOMS) pointers for the diagnostics (SYMPTOMS),
2 Good judgement (S-ENSE) all over The States (-US-) ahead of precarious (ahead of ‘Precarious’ = -P-) (SUSPENSE) uncertainty (SUSPENSE),
3 Notes (MUSIC) motorway (MI = M-I-) going around America (-US-) is top class (top ‘Class’ = -C) (MUSIC),
5 Is not excitable (EVEN-TEMPERED) in stable (EVEN-TEMPERED),
6 Do nothing (do-nothing = LAZYBONES) for zone (-Z-) in place (lay = LA-Y-) studied by osteologist (-BONES) (LAZYBONES),
7 Discontent (UNREST) in organisation? (UN-) Getting time off! (time off = -REST) (UNREST),
8 Of course (YES -), that lot on stag party (-MEN) (YES-MEN) can never say no (YES-MEN),
11 Watch (SURVEILLANCE) for clues in Ravel (clues in Ravel) medley (= anagram indicator) (clues in Ravel = SURVEILLANCE),
15 Stack’s (stacks = MOUNTAINS) horse (MOUNT-) is even magic (even letters in ‘mAgIc’ = -AI-) to school (National School = -NS) (MOUNTAINS),
16 German (BAVARIAN) voices introduce (‘Voices’ introduce = -V-) bit of Puccini, for example (-ARIA-), in bar (BA-N) (BAVARIAN),
17 One drawing the short straw perhaps (SKETCHER) gets weekend (‘weeK’ end = -K-) in Chester (Chester) mobile (= anagram indicator) (Chester = S-ETCHER) (SKETCHER),
19 Took to the boards perhaps (DANCED) in The Man (“Dan the Man” = DAN-) from Clare (-CE-) with leading director (leading ‘Director’ = -D) (DANCED),
20 Against (VERSUS) what’s in Limerick (what’s in limerick/poem = verses) by the sound of it (= homophone indicator) (‘verses’ = ‘VERSUS’),
23 It’s not a soul- (not a soul = NO ONE) -mate ultimately (‘matE’ ultimately = -E) following 1200 (12pm = noon = NO ON-)? (NO ONE).