Across:
1 Activist’s concern (CAUSE) on paper (form = FOR -M) about gangster (AL- Capone) meeting a (-A-) Russian leader (‘Russian’ leader = -R-) (CAUSE FOR ALARM) – it’s reason to be fearful (CAUSE FOR ALARM),
10 Learns a (learns a = ARSENAL) sport (= anagram indicator) from magazine (ARSENAL in military store/magazine),
11 Deposits (ALLUVIA) upstart (‘up’ start = -U-) in new (= anagram indicator) villa (villa = ALL-VI-) on Alpine summit (‘alpine’ summit = -A) (ALLUVIA),
12 Closing pages (closing ‘pages’ = S-) by the pen of old journalist perhaps (-INK) (SINK) in decline (SINK),
13 Chance to talk (SA-Y) about vegetable starter (‘vegetable’ starter = -V-) and head of (head ‘of’ = -O-) (SAVOY) cabbage (SAVOY cabbage),
15 Charges (FEES) Irish for getting out of fisheries (‘Irish’ for getting out of ‘fisheries’ = fsee = fees = FEES),
17 Sought office (RAN for office) in Lesser Antilles (in ‘Lesser Antilles’ = r An = RAN),
19 Wild cat (OCELOT) strike at foundation (‘strike’ at foundation = -E-) after (= positional indicator) college (-C-) introduces line, ominously, (introduces ‘line ominously’ = -LO-) in good books (Old Testament = O-T) (OCELOT),
21 Source of wool (ALPACA) in Alabama (AL-), Pennsylvania (-PA-) and California (-CA) (ALPACA),
22 Woe is me (TROUBLE) at the end of 36 across (at the end of ‘causes trouble’ = TROUBLE),
23 To a certain extent (IN A WAY), it’s fashionable (IN) on holiday? (away = A WAY) (IN A WAY),
25 Billy (Liam) loses the head (l) (‘Liam’ without ‘l’ = IAM-) with a fair amount of (half of) Arabic (half of ‘Arabic’ = -BIC) (IAMBIC) verse – it’s the stress! (IAMBIC),
27 Put away (ATE) in the delicatessen (in the ‘delicatessen’ = ATE),
29 Trademark Cajun sample (‘trademark Cajun’ sample = k Caj) served up (= reversal indicator) (kcaj = JACK) from the pack (JACK from pack of cards),
30 Grounds (CAUSE) for all sides today (CAUSE on all sides of today’s crossword),
31 Beam (GRIN) held by self-deprecating ringleader (held by ‘self-deprecating ringleader’ = g rin = GRIN),
34 Character’s (characters = anagram indicator) heirs (heirs = RE-ISH) welcoming doctor of divinity (doctor of divinity = -DD-) (REDDISH) visibly blushing (REDDISH),
35 Drink (HERB TEA) for bleeding heart missing Dingle (‘bleeding heart’ missing ‘Dingle’ = beheart = HERB TEA),
36 Picks a fight (CAUSES TROUBLE) with accountant (CA-) on drugs? (-USES) Bother! (TROUBLE) (CAUSES TROUBLE).
Down:
2 It makes the heart grow fonder (ABSENCE), briefly (t), being away (‘absent’ without ‘t’ = ABSEN-) from church (Church of England = -CE) (ABSENCE),
3 Ocean liner (SAND) going around Sao Tiago Island (going around ‘Sao Tiago Island’ = SAND),
4 Flighty type (FULMAR bird) from university (University of Limerick = -UL-) finds a grand (1000 = -M-) concealed in distant place (distant place = F-AR) (FULMAR),
5 Briefly (d) study (‘read’ without ‘d’ = REA-) issue (issue/offspring = -SON) (REASON) with some common sense (REASON),
6 Literally releases alert (‘literally’ releases ‘alert’ = lily = LILY) of a symbol of Easter (LILY),
7 Institute (-I-) in Shannon (RIV-ER-) close to a (-A) (RIVIERA) nice part of the world? (Nice on French RIVIERA),
8 Reason for such a slight (CAUSE OF INJURY) mint (= anagram indicator) sauce (sauce = CAUSE) with duck (O-) and piece of cod (fin = -F IN) given to peer group? (-JURY) (CAUSE OF INJURY),
9 To expose skeleton in a closet (CAUSE A SCANDAL), American (A) follows (= positional indicator) source (CAUSE) for malicious gossip (SCANDAL) (CAUSE A SCANDAL),
14 New Irish (Irish ‘new’ = -NUA-) in vessel (vat = VA-T-) heading for uninhabited for ‘uninhabited’ = -U) (VANUATU) islands in the Pacific (VANUATU),
16 Count (TOTAL) talking to missing king (‘talking to’ missing ‘king’ = talto = TOTAL),
18 Helpmate drops hem (‘helpmate’ drops ‘hem’ = lpate = PLEAT) from a piece of the skirt (PLEAT),
20 Thirsty without this (‘thirsty’ without ‘this’ = rty = TRY) to test (TRY),
21 The greatest (‘the greatest’ = Muhammad ALI) segment of lethal injection (segment of ‘lethal injection’ = al i = ALI),
24 In retrospect (= reversal indicator), Aidan (Aidan) isn’t finished (n) (‘Aidan’ without ‘n’ = aida = -ADIA) taking credit (cr = -RC-) for a (A-) (ARCADIA) pastoral setting (ARCADIA)
26 Lawyer (B-L) admits Rocky (= anagram indicator) Astor (Astor = -ORSTA-) (BORSTAL) has been asked by the judge to watch The Young Offenders (BORSTAL),
27 Battery (AA battery = AA-) for hotel (-H-) in Russia (Russia domain = .ru = -R-U-) and school (-S) (AARHUS) in Denmark (AARHUS),
28 Set (set = EST-) off (= anagram indicator) with that woman (-HER) (ESTHER) after Judith, in the Biblical sense (ESTHER follows Judith in the Old Testament),
32 Feel nostalgic for (MISS) what borders on midsummer madness (what borders on ‘midsummer madness’ = MISS),
33 Undergraduates ignoring great Dane’s (‘undergraduates’ ignoring ‘great Danes’ = urdu = URDU) language (URDU).