Across:
1 Marsh (marsh = BOG-) loses head (i) a wee bit (‘iota’ without ‘i’ = -OTA) (BOGOTA) in American capital (BOGOTA capital of Colombia),
4 They’re supposed to be better than one (TWO HEADS better than on) confusing (= anagram indicator) date show (date show = TWO HEADS),
9 Complainers have no spice (‘complainers’ have no ‘spice’ = omlanr = NORMAL) as usual (NORMAL),
10 Fault-finding (CRITICAL) electricians seen leaving (‘electricians’ ‘seen’ leaving/without ‘leaving’ = ltricia = CRITICAL),
12 Is guaranteed to work (FOOLPROOF) trick (FOOL-) in geometry class – does student have to explain it? (-PROOF) (FOOLPROOF),
13 Bar (LINE-) regularly open (‘regularly’ open = -R) (LINER) for those going abroad (LINER),
14 Be honest (TELL THE TRUTH) – elevated (= reversal indicator) accommodation (let = TEL-) left (-L) to most of them (most of ‘them’ = THE) on square (-T square) and American babe with the bat? (Babe Ruth = American baseball player = -RUTH) (TELL THE TRUTH),
18 Two of the Springs (spring = ‘well’ + spring = ‘season’ = WELL-SEASON-) losing the head (r) with revolutionary (‘red’ without ‘r’ = -ED) (WELL-SEASONED) added spice to one of those making a meal of it (WELL-SEASONED),
21 Paramilitaries (IRA-) reject (= reversal indicator) measure of intelligence (IQ = -QI) (IRAQI) from Middle East resident (IRAQI),
22 One (-I-) stuck in shapeless (= anagram indicator) entrance (entrance = NECTAR-NE) (NECTARINE) coming out of the orchard (NECTARINE),
24 Commit perjury (TELL LIES) to contradict 14 across (contradict “tell the truth” = TELL LIES),
25 Catholic (C-) hospital (-H-) for the sick (-ILL) gets unit (-I) (CHILLI) that’s a hot addition to any place where there’s consumption (CHILLI),
26 Big (LARGE-) ship (-SS-) linked to oriental (east = -E) (LARGESSE) charity (LARGESSE),
27 Link (BRIDGE) to Samuel Becket, for one, in Dublin (Samuel Becket Bridge, for one, in Dublin = BRIDGE).
Down:
1 Fundraising events (BENEFITS) peak in Scotland (BEN-) and the police will have a good look at them (e-fits = -EFITS) (BENEFITS),
2 Grey gaol (grey gaol = GARGOYLE) designed (= anagram indicator) with a source of water that’s frightening (GARGOYLE),
3 Frenchman (Messieurs = -M-) in part (part = TRA-P) upset (= reversal indicator) (TRAMP) by hike (TRAMP),
5 What someone might do to stop the fighting (WORK FOR PEACE) in Labour (WORK) is in favour of (FOR) of what the 60s dropout wanted (PEACE) (WORK FOR PEACE),
6 The thieves steal them for the getaway car (HOT PLATES) and, of course (food course), they’re responsible for added heat (HOT PLATES),
7 Type of stress (ACCENT), in a manner of speaking (ACCENT),
8 Rust (rust = SU-TR-) -free (= anagram indicator) around port (-L-) and yard (-Y) (SULTRY) – how is that not cool? (hot and humid = SULTRY),
11 They take a great interest (MONEYLENDERS) in those making advances at a ridiculous rate (MONEYLENDERS),
15 It couldn’t be further from the truth (THE BIG LIE) in most enormous Whopper (THE BIG LIE),
16 Authorised (ENTITLED) wind-up (EN-D) over ownership (-TITLE-) (ENTITLED),
17 Lippy type (EDGE-) is full of good advice (-WISE) (EDGEWISE) in Sideways (EDGEWISE),
19 Plot is (plot is = PISTOL) twisted (= anagram indicator) in the hand of the starter (in the hand of starter = starting PISTOL),
20 One of those involved in lock-in (GAOLER) could be beating out The Auld Triangle (GAOLER used metal triangle in Mountjoy Prison to wake prisoners, now immortalized in “The Auld Triangle goes Jingle Jangle …”),
23 Regard with disgust (ABHOR) how Tom leaves bathroom (‘Tom’ leaves ‘ bathroom’ = bahro = ABHOR).