Across:
8 A mechanism of controlling contagion (BUBBLE) by opening 15 across (opening 'bubble and squeak' = BUBBLE),
9 One of those arriving early in the year (AQUARIAN arrives in January) for drink (AQUA-) and atmosphere (air) around (= reverse indicator) (air = -RIA-) first night (first ‘Night’ = -N) (AQUARIAN),
10 Whopping great (HUGE) sides on the Humber Bridge (sides on the ‘HUmber bridGE’ = HUGE),
11 Awful (= anagram indicator) sentiment (sentiment = INTE-MENTS) about never-ending (‘neveR’ ending = -R-) (INTERMENTS) grave procedures? (INTERMENTS),
12 Salesperson cuts pears (‘salesperson’ cuts ‘pears’ = sleson = LESSON) for the students to digest? (LESSON),
14 Earn (earn) treat (= anagram indicator) (earn = NEAR) and present (HERE) (NEAR HERE) from next door (NEAR HERE),
15 Effervescence (BUBBLE) and heads on ales noted (heads on ‘Ales Noted’ = AN-) for depth (-D) to go with what sounds like unoiled (SQUEAK) (BUBBLE AND SQUEAK), fried leftovers! (BUBBLE AND SQUEAK),
18 Men preparing to join Union (fiancés = FI-ANCES) in possession of new (-N-) (FINANCES) funds (FINANCES),
20 Tidies up (CLEANS) to do some homework (CLEAN-) at school (-S) (CLEANS),
22 Do it fine in (do it fine in) sporty (= anagram indicator) (do it fine in = DEFINITION) Focus (DEFINITION),
24 Foremost calories (foremost ‘Calories’ = -C-) in butter? (fat = FA-T) (FACT) Every scientist knows that! (FACT),
25 Labs seem (labs seem) unstable (= anagram indicator) (labs seem = ASSEMBLE) to construct (ASSEMBLE),
26 Tweet (SQUEAK) at the end of 15 across (at the end of ‘bubble and squeak’ = SQUEAK).
Down:
1 Is always the last to arrive (FUTURE), as it's never discussed in history class (FUTURE),
2 Allowed to (ABLE) get around (= reversal indicator) Mediterranean island (Elba = ABLE),
3 Refuse to obey (DEFIANCE) prospective partner (fiancé = -FIANCE) after (= position indicator) short (n) study (den) (‘den’ without ‘n’= DE-) (DEFIANCE),
4 Harry (harry/harass = BAIT) is the first person (-I-) welcomed by cricket club (BA-T) (BAIT),
5 Local enthusiast (NUT-) from academy (Royal Irish Academy = -RIA) (NUTRIA) in fur (NUTRIA 6 Politician (GREEN-) from Dáil Éireann (-HOUSE) (GREENHOUSE) is sowing the seeds of something big perhaps (GREENHOUSE),
7 Turns up (= reversal indicator) broadcast (air = -RIA), etc., (etc = -CTE-) on an old boat (able-bodied seaman = ab = BA-) (BACTERIA) with bugs (BACTERIA),
13 Those crafty types under a lot of pressure (SUBMARINES) in public transport (bus) upset (= reverse indicator) (bus = SUB-) going to unusual (= anagram indicator) seminar (seminar = MARINES) (SUBMARINES),
14 Sounds like (= homophone indicator) an excuse for not being able to distinguish wines (‘no nose’ ‘no nos’ = NO-NOS) in Prohibition and such like (NO-NOS),
16 Really large space (UNIVERSE) in a foreign (‘a’ foreign = UN-) institute (-I-) near Limerick (-VERSE) (UNIVERSE),
17 Well, it’s not (SICKNESS) a complaint you’d bring to your doctor (SICKNESS),
19 Goes up Croagh Patrick (CLIMBS) with 100 (C-) members (-LIMBS) (CLIMBS),
21 In the neighbourhood (near = NE-AR) around court (-CT-) (NECTAR) for a delicious drink (NECTAR),
23 Some reggae – divine on reflection (= reverse indicator) (some ‘reggAE – DIvine’ on reflection = aedi = IDEA), in theory (IDEA),
24 Some subdue fighters raised (= reverse indicator) (some ‘subDUE Fighters’ = duef = FEUD) in bitter quarrel (FEUD).