Across:
1 Concealed (HIDDEN) or opened by 14 across? (opened by ‘hidden agenda’ = HIDDEN),
4 Is capable of modifying (ADAPTIVE) IT pad (it pad) picked up (= reverse indicator) (it pad = -DAPTI-) in Avenue (A-VE) (ADAPTIVE),
9 Manage (AFFORD) a (A-) firm opening (‘Firm’ opening = -F-) with access across the river (crossing place in a river = -FORD) (AFFORD),
10 Visitor (caller = CALL-ER) admits address (-IP- address) (CALLIPER) is helpful for someone who wants to go for walks (CALLIPER),
12 American (A-) method (-WAY) for starting fire (MATCH) (AWAY MATCH) with a fixture that’s a distance from home (AWAY MATCH),
13 Alludes to (alludes/refers to = CITES) Washington and Lincoln, for instance (cities), not taking the fourth? (‘i’) (‘cities’ without ‘i’ = CITES),
14 An ulterior motive (HIDDEN AGENDA) for what hide-and-go-seek champion might have up his sleeve (HIDDEN AGENDA),
18 Irish (LANGUAGE) soul mate (soul mate in ‘body and soul’ = BODY) starts (= position indicator) (BODY LANGUAGE) shrugging shoulders (BODY LANGUAGE),
21 It’s soul-destroying without getting on? Soldier (‘soul destroying’ without getting ‘on soldier’ = sutyg = GUTSY) is courageous! (GUTSY),
22 Found (ESTABLISH) stable’s (stables) jockey (= anagram indicator) (stables = ESTABL-S) holding his heart (‘hIs’ heart = -I-) on top of horse (top of ‘Horse’ = -H-) (ESTABLISH),
24 Defeatist (NEGATIVE) never gives the positive side of the picture (NEGATIVE),
25 Render defenceless (DISARM) with Garda’s (DI’s = DIS-) gun (-ARM) (DISARM),
26 One crying wolf (ALARMIST) as a mistral (a mistral = ALARMIST) blows up? (= anagram indicator) (ALARMIST),
27 Turns up (= reverse indicator) after the deadline (late = E-TAL) around mid-evening (mid ‘eveNing’ = -N-) armed ultimately (‘armeD’ ultimately = D-) (DENTAL) to the teeth, as the description goes (DENTAL).
Down:
1 Annoying situation (HEADACHE) as big cheese (HEAD-) on starter of antipasti (starter of ‘Antipasti’ = -A-) isn’t finished (w)? Take a mouthful! (‘chew’ without ‘w’ = -CHE)! (HEADACHE),
2 Doctor (= anagram indicator) Freud (Freud = DEFR-U-) takes in a (-A-) detective (-DS) (DEFRAUDS) and cons (DEFRAUDS),
3 It’s more (more) about rejecting (= reverse indicator) (more = E-ROM) primarily photographic (primarily ‘Photographic’ = -P-) (EPROM) memory – is that very PC? (computing read-only memory = EPROM),
5 Life partner (Life and DEATH) gets fine (PENALTY) (DEATH PENALTY) – it’s the severest of punishments for someone who’s going to be late (DEATH PENALTY),
6 Sniffs drugs on the job (POLICE DOG) working on a lead in the law enforcement business (POLICE DOG),
7 Attribute (IMPUTE) to independent (I-) foreign politician (-MP-) – a Native American (-UTE) (IMPUTE),
8 Serge’s (serges) adaptation (= anagram indicator) (serges = EGRESS) is way out (EGRESS),
11 Support (STEADY) butterflies (NERVES) (STEADY NERVES) to put your mind at rest (STEADY NERVES),
15 English (E-) officer (Commanding Officer = -CO-) network (-SYSTEM) (ECOSYSTEM) for community in the wild (ECOSYSTEM),
16 Costume in The Abbey (HABIT-) for lead actor (lead ‘Actor’ = -A-) on opening night (opening ‘Night’ = -N-) from time (-T) (HABITANT) of an old French settler in Quebec (HABITANT),
17 British (B-) swimmer (eel = -E-EL) consumes cold (-C-), cured meat (-HAM-) (BÉCHAMEL) in sauce (BÉCHAMEL),
19 To-do list (AGENDA) at the end of 14 across (at the end of ‘hidden AGENDA’ = AGENDA),
20 Pragmatist will have no part (‘pragmatist’ will have no ‘part’ = gmaist = STIGMA) in disgraceful sign (STIGMA),
23 Pay off (BRIBE) Josh (josh/tease = -RIB-) in Belgium (B-E) (BRIBE).