Across:
1 In a panic (SCARED) showing off (= anagram indicator) cedars (cedars = SCARED),
4 Sick (sick = IC-KS) characters (= anagram indicator) welcome heroic (-E PIC-) (ICE PICKS) blockbusters sold to those with a mountain to climb (ICE PICKS),
9 Short, crisp style (ABRUPT) of alcohol primarily (‘Alcohol’ primarily = A-) from dry wine (brut = -BRU-T) including Port (-P-) (ABRUPT),
10 One (I-) foreign politician (-MP-) took to the stage (-ACTED) (IMPACTED) and strongly had an influence on The Crash? (IMPACTED on ...),
12 Lack of harmony (IMBALANCE) in ambiance (ambiance) all over the place (= anagram indicator) (ambiance = IMBA-ANCE) around central Ireland (central ‘IreLand’ = -L-) (IMBALANCE),
13 Denies (denies) one (i) is missing (‘denies’ without ‘i’ = denes = NEEDS) essentials (NEEDS),
14 Secretary’s boss (DICTATOR-) is a crafty type (-SHIP) (DICTATORSHIP) responsible for keeping the people under the thumb as a rule (DICTATORSHIP),
18 Come to terms with (MASTER) Frost (Frost) reworking (= anagram indicator) (Frost = OF -RTS) around “Away!” (A-) (MASTER OF ARTS) to a degree (MASTER OF ARTS),
21 Mature (ADULT) soldier (lieutenant = -LT) follows (= position indicator) revolutionary (= reversal indicator) paramilitaries (Ulster Defence Association = UDA = ADU-) (ADULT),
22 Use external company (OUTSOURCE) to expose (OUT-) spy’s confidant (-SOURCE) (OUTSOURCE),
24 Private (secret = SECRET-) left (-L-) yard (-Y) (SECRETLY) on the sly (SECRETLY),
25 23 down (‘offer’ = OFFER-) getting endless (end ‘lesS’ = -S) (OFFERS) bargains (OFFERS),
26 Adult tie (adult tie) designed (= anagram indicator) (adult tie = LATITUDE) for freedom of movement (LATITUDE),
27 Shade missing from Harris Tweed (‘shade’ missing from ‘harris tweed’ = rritwe = WRITER) – that’s one for the books (WRITER).
Down:
1 Notices (sees) report (= homophone indicator) (‘sees’ = ‘seas’ = SEA -S) about Robin (BIRD-) (SEA BIRDS) and The Penguin’s group (the penguin’s group = SEA BIRDS),
2 Exercises (AEROBICS) for a scribe (a scribe = AER-BICS) working (= anagram indicator) around Oxford (-O-) (AEROBICS),
3 Kick out (EXPEL) former (EX-) pupil (learner = -L) behind (= position indicator) a fair amount of (half of) the nuisance (pest) (half of ‘PEst’ = -PE-) (EXPEL),
5 Brief (t) note (‘comment’ without ‘t’ = COMMEN-) from us (US) reviewed (= reverse indicator) (US = -SU-) by judge (-RATE) (COMMENSURATE) or the equivalent (COMMENSURATE),
6 From flat (plane) we hear (= homophone indicator) (‘plane’ = ‘plain’ = PLAIN-) row (-TIFF) (PLAINTIFF) – the one that’s bound to be heard by the court (PLAINTIFF),
7 Column inches, leaders (‘Column Inches’ leaders = CI-) and links (-TIES) (CITIES) to Washington and Moscow (CITIES),
8 Samuel (SA-M) conceals Diana’s (Di’s = -DIS-) (SADISM) humiliating cruelty (SADISM),
11 A foreign (a = UN-) criminal (-CON-) insulted others online (-TROLLED) (UNCONTROLLED) without restraint (UNCONTROLLED),
15 A Room With a View perhaps (APARTMENT) produced by Studio (studio APARTMENT),
16 Obsessively indecent type (PRURIENT) produces pressure (P-) on Russia (-RU-) to find individual (-I-) in Split (split/tear = -R-ENT) (PRURIENT),
17 Señorita (lass) heading off (l) (‘lass’ without ‘l’ = ASS-) to Roses (roses) working (= anagram indicator) (roses = -ESSOR) (ASSESSOR) for The Examiner (ASSESSOR),
19 It relates to reason (CAUSAL) for employing (= anagram indicator) casual (casual = CAUSAL),
20 American state (California = -CA-) consumed by unfermented grape juice (must = MUS-T) (MUSCAT) from Asian capital (Capital city of Oman = MUSCAT),
23 Proposal (OFFER) for volunteer? (OFFER).