Across:
8 A sin leaving East Indian ('a sin' leaving 'East Indian' = etdian = DETAIN) to be put behind bars (DETAIN),
9 Did a job on the numbers (ARRANGED) for a grander (a grander) model (= anagram indicator) (a grander = ARRANGED),
10 A large beastly growth (beastly/animal growth = HORN) in English ornaments (in ‘englisH ORNaments’ = HORN),
11 Horticulturally, borders (HorticulturallY’ borders = HY-) with flower (flower/river -PO-) from 19 down? (‘thesis’ = -THESIS) (HYPOTHESIS) As a proposition, this needs to be tested! (HYPOTHESIS),
12 Grapes primarily (‘Grapes initially = G-) from bottom of the slopes (bottom of the ‘slopeS’ = -S-) in unpleasantly damp and cold (dank = -DAN-K) (GDANSK) port (port in Poland = GDANSK),
14 It’s not easy to learn (HARD-) craft (-SHIP) (HARDSHIP) in Dire Straits (HARDSHIP),
15 Backed down from a fight (DREW IN ONE’S HORNS) with 24 across (‘drew’ = DREW) to get in (IN) with 23 down (‘ones’ = ONE’S) and 10 across (‘horn’ = -HORN-) at school (-S) (DREW IN ONE’S HORNS),
18 Ed – stupid (ed stupid), foolish (= anagram indicator) (ed stupid = DISPUTED) and challenged? (DISPUTED),
20 Daughter (D-) is unfamiliar (-ODD-) with the foreign (‘the’ foreign = -LE) (DODDLE) piece of cake (DODDLE),
22 Training hub for The Boys in Blue (TEMPLEMORE Garda Training College) and for those worshipping their idols (TEMPLE-), to a greater extent (-MORE) (TEMPLEMORE),
24 Pulled out a piece (DREW a piece/gun) of 24 down in the past (‘draw’ in the past = DREW),
25 Headache (MIGRAINE) for Frenchman (Monsieur = M-) travelling (= anagram indicator) Nigeria (Nigeria = -IGRAINE) (MIGRAINE),
26 First Lady (EV-E-) entertaining Bill (-AD-) and daughter (EVADED) gave someone a wide berth (EVADED).
Down:
1 Is free of (DEVOID of) backward (= reverse indicator) journalist (Ed = DE-) opposing (-V-) old (-O-) papers (-ID papers) (DEVOID),
2 Up (= reverse indicator) north (NI = -IN), a (-A-) sign (V- sign) (VAIN) of what’s conceited? (VAIN),
3 Is not disturbed (UNSHAKEN) by organisation (United Nations = UN-) getting upset (-SHAKEN) (UNSHAKEN),
4 Shines a light (LAMP) on the extremes of labour camp (on the extremes of ‘LAbour caMP’ = LAMP),
5 Town crier dismisses Con (‘town crier’ dismisses ‘Con’ = twrier = WRITER) Joyce, for one (James Joyce, for one = WRITER),
6 Got it (UNDERSTOOD) from Roger? (UNDERSTOOD),
7 Judge’s verdict (DECISION) ridiculously (= anagram indicator) is in code (is in code = DECISION),
13 Information (NEWS-) on official documentation (-PAPERS) (NEWSPAPERS) in rags? (NEWSPAPERS),
14 Mate got off methadone (‘mate’ got off ‘methadone’ = hdone = HONED) and improved (HONED),
16 Checked (REINED IN) with a German (‘a’ German = -EINE-) in research and development (R&D = R-D) that it’s on trend (IN) (REINED IN),
17 Dispirited (SADDENED) by enigmatic (= anagram indicator) dead ends (dead ends = SADDENED),
19 Article (THE-) on MI6 (Secret Intelligence Service/ also known as MI6 = -SIS) (THESIS) produced by one at university (THESIS),
21 Males from Emerald Isle (‘males’ from ‘Emerald Isle’ = erdile = LIEDER) produced German songs (LIEDER),
23 Individuals (ONES) in India abandoned by Indonesia (‘India’ abandoned by ‘Indonesia’ = ones = ONES),
24 Part of the hospital (ward) set up (= reverse indicator) (ward = DRAW) by local Yank (yank/pull = DRAW).