Across:
8 Democracies missing codes (‘democracies’ missing ‘codes’ = mracie = CRIMEA) in Ukraine, or is it Russia? (CRIMEA),
9 Lively type (ANIMATED) made it an (made it an = ANIMATED) order (= anagram indicator),
10 Leave one’s mark on something (SIGN) similar to 2 down (‘omen’ is similar to SIGN),
11 They’ll get you thinking (BRAIN CELLS) of most of those from 1 down (most of ‘brains’ = BRAIN) in prison (CELLS) (BRAIN CELLS),
12 Oops (oops = -SOPO-) – criminal (= anagram indicator) in papers (ID papers = I-D) (ISOPOD) is a little crawler (ISOPOD),
14 Opens 15 across (opens ‘brain specialist’ = BRAIN-) to fight (-BOX) (BRAINBOX) intelligent character (BRAINBOX),
15 Expert attending to 11 across (expert attending to ‘brain cells’ = BRAIN SPECIALIST) knows a swelled head when they see it (BRAIN SPECIALIST),
18 Bed (-COT-) is large (-L-) in ocean liner (ocean liner = S-AND) (SCOTLAND) produced in the UK (SCOTLAND),
20 Sounds like (= homophone indicator) the Arabians perhaps (Arabian horses = ‘mares’ = ‘MAYORS’) running a number of cities (MAYORS),
22 Her career’s (her careers = RESEARCHER) disrupted (= anagram indicator) by investigator (RESEARCHER),
24 Bad-mannered (RUDE) crusader dismisses scar (‘crusader’ dismisses ‘scar’ = rude = RUDE),
25 Making the most sense (BRAINIER) of leader in 15 across (leader in “brain specialist” = BRAIN-) losing the head (p) on waterfront (‘pier’ without ‘p’ = -IER) (BRAINIER),
26 View (ASPECT) of a (A-) small (-S-) leaderless (O) bunch of oil producers (‘OPEC’ without ‘O’= -PEC-) from half of Texas (half of ‘TX’ = -T) (ASPECT).
Down:
1 Minds (BRAINS) what’s covered in algebra in school (what’s covered in ‘algebra in school’ = BRAINS),
2 Some of the ghetto mentality (some of the ‘ghetto mentality’ = OMEN) can be read as a warning (OMEN),
3 Graduate (-BA-) party (-DO-) in public houses (BAR-S) (BARBADOS) for the islanders (BARBADOS),
4 Great Dane releases Green (‘great dane’ releases ‘green’ = atda = DATA) facts and figures (DATA),
5 Anne IV (Anne IV = VIENNA) retires (= reversal indicator) to European city (VIENNA),
6 Typical of a mother (MATERNALLY) and wife (MATE-) to mobilise (-R-ALLY) over last altercation (last ‘altercation’ = -N-) (MATERNALLY),
7 Old magazine (BELL-) section in department store (-BOYS) (BELLBOYS) for runners in American hotels (BELLBOYS),
13 Misfit (= anagram indicator) painted rim (painted rim = PRINT MEDIA) of press (PRINT MEDIA),
14 Side bar is missing (‘side bar’ ‘is’ missing/without ‘is’ = debar = BREAD) something to put the butter and marmalade on (BREAD),
16 Produces the hits that are out of this world (rocket science = ROCKETRY) for those aiming for the stars (ROCKETRY),
17 One (I-) Frenchman (Monsieur = -M-) is a type of enemy (-MORTAL enemy) (IMMORTAL) here to stay perhaps (IMMORTAL),
19 American (A-) loud music (forte = -F-) – Rob (rob = -RAID) (AFRAID) is shaking in his boots (AFRAID),
21 Edit (REDUCE) crude (Crude = REDUC-) novel (= anagram indicator) giving it direction (east = -E) (REDUCE),
23 Douglas leaves shoulder bag (‘Douglas’ leaves ‘shoulder bag’ = herb = HERB) for Rosemary (HERB),
24 Included by retrograde (= reversal indicator) software pirates (included by retrograde ‘softwarepirates’ = epir = RIPE) – the mature variety (RIPE).