Across:
8 Follows Spring (SUMMER) in some (some) state (= homophone indicator) ('some' = 'sum' = SUM-) to poetic lake (mere) briefly (e) ('mere' without 'e' = -MER) (SUMMER),
9 Eagerness (ALACRITY) for American (A-) credit (-CR-) in volatile (= anagram indicator) Italy (Italy = -LA-ITY) (ALACRITY),
10 and 4 down: Private place (BOOT CAMP) in Wellington (Wellington BOOT) College (C-) for a (-A-) foreign politician (-MP) (BOOT CAMP),
11 No pilau pot (no pilau pot) dish (= anagram indicator) (no pilau pot = POPULATION) for all the people (POPULATION),
12 Pointless releasing tip (‘pointless’ releasing ‘tip’ = onless = LESSON) or something that will appeal to students (LESSON),
14 Irish men (Irish ‘men’ = FIR-) find online auction (e-sale = -E SALE) (FIRE SALE) of goods that were once really hot perhaps (FIRE SALE),
15 Bootleg singles (CRIMINAL RECORDS) produced by The Yardbirds during their career (CRIMINAL RECORDS),
18 That man (-HE-) in Clones (Clones) working (= anagram indicator) (Clones = EC-LONS) (ECHELONS) with levels of authority (ECHELONS),
20 It’s not hard (easy = EAS-Y) consuming heads of iceberg lettuce (heads of ‘Iceberg Lettuce’ = -IL-) (EASILY) without effort? (EASILY),
22 Is due to get married (HEN-) to Clare (-CE-) Knox? For one (-FORT- Knox), that’s going to get hot (H) (HENCEFORTH) from now on! (HENCEFORTH),
24 In part, consider a venture (in part ‘consideR A VEnture’ = RAVE) that appeals to dancers (RAVE),
25 Banned (CRIMINAL) for opening 15 across (opening ‘criminal records’ = CRIMINAL),
26 Album (RECORD) by New (= anagram indicator) Order (order = RE-ORD) includes first cover (first ‘Cover’ = -C-) (RECORD).
Down:
1 Asian neighbour (EUROPE) provides a home for Irish and French citizens, for example (both part of the continent of EUROPE),
2 Don’t include (OMIT) what’s upsetting (= reverse indicator) in testimony (in ‘tesTIMOny’ = timo = OMIT),
3 Doctor (DR-) takes position (opinion) without one (i) (‘opinIon’without ‘i’ = opinon = -OP IN ON) (DROP IN ON) visit (DROP IN ON),
4 See 10 across,
5 One of those involved in lock-in (JAILER) is a key figure? (JAILER),
6 Ross Potter (Ross Potter) rioting (= anagram indicator) (Ross Potter = PROTESTORS) with those marching with the mob (PROTESTORS),
7 Took time walking (STROLLED) from port to port (port = left x 2 = -LL-) in busy (= anagram indicator) Dorset (Dorset = STRO-ED) (STROLLED),
13 8 across (‘summer’ = SUMMER) and 4 down (‘camp’ = CAMP) (SUMMER CAMP) is open for the kids during the holidays (SUMMER CAMP),
14 Works in the office (FILES) for Life (life = FILE-) Style (= anagram indicator) Sports from the beginning (‘Sports’ from the beginning = -S) (FILES),
16 Music (ROCK-) to have a go at (-TRY) after (= position indicator) closing time (closing ‘timE’ = -E-) (ROCKETRY) utilises science that’s bound to take off (ROCKETRY),
17 Person with a lot of contacts (optician) asked me to look at this? (optician asked me to look at EYE CHART) If you get to view it, you’re probably seeing things! (EYE CHART),
19 Switch (switch OFF-) at Terminus (-END) (OFFEND) or don’t get on at all (OFFEND),
21 Takes up (= reverse indicator) your (yr = -RY) wicked (evil = LIVE-) (LIVERY) uniform (LIVERY),
23 Winning Streak (roll), it’s said (= homophone indicator) (‘roll’ = ‘ROLE’), has a part to play (ROLE),
24 Stack in a farmer’s field (RICK) mile from Limerick (‘mile’ from ‘Limerick’ = rick = RICK).