Across:
1 What was the function (SOCIAL) of The Gathering? (SOCIAL),
4 Labs seem (labs seem) uncomplicated (= anagram indicator) (labs seem = ASSEMBLE) to construct (ASSEMBLE),
9 In the autumn (SEPT-), one (-I-) cold (-C) (SEPTIC) could be infectious (SEPTIC),
10 Turns up (= reverse indicator), Arab prince (Emir = -RIME) is cold (C-), crude (raw = WAR) (WAR CRIME) and in violation of the Geneva Convention (WAR CRIME),
12 A tune Gore (a tune Gore) played (= anagram indicator) (a tune Gore = ENTOURAGE) to followers (ENTOURAGE),
13 The sun is responsible for this (LIGHT) scourge for farmers (blight) starting off (b) (‘blight’ without ‘b’ = LIGHT),
14 Type of pitch (SALES pitch) for football coach (MANAGER) (SALES MANAGER) – the one responsible for the team’s results after making the most of promotion (SALES MANAGER),
18 Has a mind to study (PSYCHIATRIST) one of those professions that go over some peoples’ heads perhaps (PSYCHIATRIST),
21 Swinger’s type (HINGE) of joint? (HINGE).
22 Formal account of The Epistles, for example (STATEMENT), in New (= anagram indicator) Testament (testament = STATEMENT),
24 Cured (REMEDIED) starters of red, essential meats enwrapped (starters of ‘Red Essential Meats Enwrapped’ = REME-) and packed up? (-DIED) (REMEDIED),
25 Síle (sile) gets up (= reversal indicator) (sile = -ELIS-) after (= position indicator) writing (from the 3Rs = R-) to hotel (-H) (RELISH) with gusto (RELISH).
26 Sly idiot (sly idiot) confused (= anagram indicator) (sly idiot = SOLIDITY) by firm state (SOLIDITY),
27 Lost the head (f) – went for it (‘fetched’ without ‘f’ = ETCHED) and left a lasting impression (ETCHED).
Down:
1 Hearing (S-ENSE) covers up marketing ploy (Unique Selling Point = -USP-) (SUSPENSE) – it leads to nervous uncertainty (SUSPENSE),
2 A plastic (a plastic) explosive (= anagram indicator) (a plastic = CAPITALS produced by Moscow and Beijing (examples of CAPITALS),
3 Farewell (ADIEU) to colourful type (dye) on the radio (= homophone indicator) (‘dye’ = ‘die’ = -DIE-) in Australia (A-U) (ADIEU),
5 Small (S-), fast runner (-HARE) from Nairobi, for example (Nairobi, for example, = CAPITAL of Kenya) (SHARE CAPITAL), in Equity? (SHARE CAPITAL),
6 Former (EX-) church (Church of England = -CE-) novice (learner = -L-) provided credit (-LENT) (EXCELLENT) for what’s outstanding (EXCELLENT),
7 Connection (BRIDGE) to Samuel Beckett in Dublin, for instance (Samuel Beckett BRIDGE),
8 Returns (ELECTS) to select (select) ground (= anagram indicator) (select = ELECTS),
11 Steady (BALANCE) on? The Paper (SHEET) (BALANCE SHEET) has financial statement that includes 5 down! (BALANCE SHEET includes ‘share capital’),
15 American writer (US pen = -USPEN-) upsets (= reverse indicator) editor (ed = -DE-) in South Dakota (South Dakota = SD = S-D) (SUSPENDED) temporarily out of work (SUSPENDED),
16 One of The Greens (SIDE DISH) produced Dublin (SIDE) coddle (DISH) (SIDE DISH),
17 Hemmed (STITCHED) in small songbird (-TIT-) caught (-C-) in outbuilding (shed = S-HED) (STITCHED),
19 Goes fifty-fifty (SHARES) with more than one starting 5 down (more than one starting ‘share capital’ = SHARES),
20 Dine off Indian meal (‘dine’ off ‘Indian meal’ = inamal = ANIMAL) and fish (ANIMAL),
23 Do Trojan work (EXERT) for mystery man (-X-) hidden by Hazel (hazel tree = E-ERT) Back (= reverse indicator) (EXERT).