Across:
1 Organisation (SYSTEM) produces mass hysteria dismissing Sharia (‘mass hysteria’ dismissing ‘Sharia’ = mssyte = SYSTEM),
4 In France, John (John = Jean) isn’t finished (n) (‘Jean” without ‘n’ = JEA-) with awful (-LOUSY) (JEALOUSY) resentment (JEALOUSY),
9 Fails to notice (MISSES) one (-I-) school (-S-) in frightful (= anagram indicator) mess (M-SES) (MISSES),
10 Fits (SEIZURES) what the Customs are responsible for (SEIZURES),
12 Pick up the tab (TREAT-) for workers (-MEN-) first to talk (first to ‘talk’ = -T) (TREATMENT) in therapy (TREATMENT),
13 Roughly stitch up (loose stitches = BASTE) graduate (BA-) joining The Branch (stem) shortly (m) (“stem” without “m” = -STE) (BASTE),
14 Type of production (MASS production) from the symphony (MOVEMENT) (MASS MOVEMENT) in March (march = MASS MOVEMENT),
18 Position (BUSINESS) on pitch (PARK) (BUSINESS PARK) provides latitude for those with concerns (BUSINESS PARK),
21 Representative (AGENT) of a (A-) fairly (half of) hard-working type (half of ‘diligent’ = -GENT) (AGENT),
22 A (A-) man (-ERIC-) overshadowed by another man? (-M-AN-) Old Bob (old bob/shilling = -S) (AMERICANS) in The Bostonians! (AMERICANS),
24 Line (COR-D-) about most of (p) the last letters (‘RIP’ without ‘p’= -RI-) from old (-O-) writing (from the 3Rs = -R) (CORRIDOR) passage (CORRIDOR),
25 Bill (AD-) is at the bar (Attorney General = -AG-) before 10 (-IO) (ADAGIO) – that’s slow for a musician (ADAGIO),
26 It’s an utter failure (DISASTER) to set up (= reversal indicator) Sidney (Sid = DIS-) with Daisy’s relative (-ASTER) (DISASTER),
27 You could read this to mean I’m challenging you to a run? (‘race me’ = ‘RACEME’) That would be some blooming arrangement! (a flower head = RACEME).
Down:
1 Theatrical type will recognise these clues (SYMPTOMS recognised by theatrical type/doctor) could be signs of a greater malaise (SYMPTOMS),
2 Hearing (S-ENSE) covers up marketing ploy (unique selling point = -USP-) (SUSPENSE) – it leads to nervous uncertainty (SUSPENSE),
3 Standing up (ERECT) to selectors dismissing loss (‘selectors’ dismissing ‘loss’ = eectr = ERECT),
5 Is not excitable (EVEN-TEMPERED) in stable (EVEN-TEMPERED),
6 The flâneurs (LAZYBONES) won’t do anything (LAZY-) with The Mandibles (mandibles/jawbones = -BONES) (LAZYBONES),
7 AGM comes out of arguments (‘AGM’ comes out of ‘arguments’ = ruents = UNREST) and trouble (UNREST),
8 Alarm (= anagram indicator) enemy (enemy = YE-MEN) incorporating endless (end ‘less’ = -S-) (YES-MEN) sycophants (YES-MEN),
11 Steer clear of (LEAVE IT ALONE) action when it doesn’t concern you (LEAVE IT ALONE),
15 Stack’s (stacks = MOUNTAINS) horse (MOUNT-) on a (-A-) popular (-IN-) racecourse is second last (‘racecourse’ is second last = -S) (MOUNTAINS),
16 Hear a gun (hear a gun = HARANGUE) going off (= anagram indicator) in lecture (lecture/rant = HARANGUE),
17 Most of (n) the film (‘skin’ without ‘n’ = SKI) pitch (SLOPE) (SKI SLOPE) has a tendency to go downhill (SKI SLOPE),
19 Had a ball perhaps (DANCED) in The Man (“Dan the man” = DAN-) from Clare (-CE-) with leading director (leading ‘director’ = -D) (DANCED),
20 Zoo opening (‘zoo’ opening = Z-) with wild (= anagram indicator) bears (bears = EBRAS) (ZEBRAS) – the black and white ones that go around in bands (ZEBRAS have black and white bands),
23 Country (INDIA) covered up trade in diamonds (‘trade in diamonds’ = in dia = INDIA).