Across:
1 Ascertaining the time (DATING) for those looking to join Union (DATING),
4 Ready to go with (SET-) supports (-BACKS) (SETBACKS) out of disappointments (SETBACKS),
9 No matter – it’s not part of shock treatment (‘no matter’ it’s not part of ‘shock treatment’ = shckte = SKETCH) by design (SKETCH),
10 Heads of the old sections (heads ‘Of The Old Sections’ = OTOS -) manage (-COPE-) (OTOSCOPE) one investigating The Hearing (OTOSCOPE),
12 The Commission (BROKERAGE) is cleaned out (BROKE-) by bank’s rate of exchange (banks ‘RAte of exchanGE’ = -RAGE) (BROKERAGE),
13 Starter of antipasti (starter of ‘Antipasti’ = A-) with some green salad (-HEAD of lettuce) (AHEAD) in front of others (AHEAD),
14 Is unaffected by the elements (WEATHERPROOF) in, for example, global warming (WEATHER-) evidence (-PROOF) (WEATHERPROOF),
18 Determines the age of sample (CARBON DATING) in 25 across (‘carbon’ = CARBON) and 1 across (‘dating’ = DATING) (CARBON DATING),
21 Loses all sides (‘p’ + ‘i’) in patolli (‘patolli’ without ‘p + i’ = ATOLL) to Rosalind Bank, for example! (Rosalind Bank ATOLL in the western Caribbean Sea),
22 Foreign (= anagram indicator) niece’s mum (niece’s mum = ECUMENISM) produces philosophy in favour of religious unity (ECUMENISM),
24 Rebel (RENEGADE) in criminal (= anagram indicator) den? Agree! (den agree = RENEGADE),
25 Copy that (CARBON copy/paper) ale from Barcelona? (‘ale’ from ‘Barcelona’ = brcona = CARBON)
26 Lawyer (Senior Council = SC-) has right (-R-) to breakdown (= anagram indicator) Cathy (Cathy = -ATCHY) (SCRATCHY), causing friction, for the record (friction/noise on the record/LP = SCRATCHY),
27 One of the Greens (pea = PE-A) possesses place (-SET-) (PESETA) in old European capital (capital/money = PESETA).
Down:
1 Work in the house (DUST) on dish (BOWL) (DUST BOWL) from drought-stricken area (DUST BOWL),
2 Driver’s aid, of course (TEE-), for getting the entire amount (-TOTAL) (TEETOTAL) on the wagon (TEETOTAL),
3 Ideal position (NICHE) for hotel (-H-) in Mediterranean city (Nice = NIC-E) (NICHE),
5 The risk-taking, moneymaking producer (ENTREPRENEUR) of The Merchant of Venice, for instance (ENTREPRENEUR),
6 A piece of dance music (BOSSA NOVA) to master (BOSS-) for a (-A) Radio station (Radio NOVA) (BOSSA NOVA),
7 Hutch (hutch/pen = COOP-) and Ward (ward = -ER) (COOPER) running off barrels of porter perhaps (COOPER makes barrels/casks),
8 Stayed (stayed) out (= anagram indicator) (stayed = STEADY) of stable (STEADY),
11 Little space between two parts (HAIR’S BREADTH) of locks (HAIRS) for feeding the ducks (BREAD-) on a fair amount of (half of) the path (half of the ‘paTH’ = -TH) (HAIR’S BREADTH),
15 From top to bottom (height = HE-IGHT) around summit of Aghla (summit of ‘Aghla’ = -A-), Donegal (DL = -DL-) (HEADLIGHT), you can see it coming from the car, in the dark (HEADLIGHT),
16 Is not to be envied (PITIABLE) going up (= reverse indicator) small Mediterranean island (Elba = -ABLE) to one (-I-) peak (tip = PIT-) (PITIABLE),
17 Too much confidence in one’s own depiction (EGOMANIA) in image on a (image on a) building (= anagram indicator) (image on a = EGOMANIA),
19 Rogue (CAD-) leaders of Roman Empire (leaders of ‘Roman Empire’ = -RE-) from small (-S) (CADRES), small groups (CADRES),
20 Roadie from Rio de Janeiro (‘roadie’ from ‘Rio de Janeiro’ = iejnro = JOINER) for one of The Carpenters (JOINER),
23 Put a line through (ERASE) some disclaimer as expected (some ‘disclaimER AS Expected’ = ERASE).