Across:
8 In the market (TRADER) for square (T- square) pirate ship (raider) with no third sail (third ‘saIl’ = i) (‘raider’ without ‘i’ = -RADER) (TRADER),
9 Show business (DRAMATIC) by opening 15 across (opening ‘dramatic setting’ = DRAMATIC),
10 Leave out (OMIT) what’s upsetting (= reverse indicator) in testimony (in ‘tesTIMOny’ = timo = OMIT),
11 State (BULGARIA-) school (National School = -NS) (BULGARIANS) for those living in Eastern Europe (BULGARIANS),
12 Concerns Will (ESTATE) and Earl (E-) from Oklahoma (-STATE) (ESTATE),
14 Mollycoddled (SPOON-FED) a gent (‘a gent’ = ‘agent’ = Fed = -FED) behind (= position indicator) one causing a stir (SPOON-) (SPOON-FED),
15 Those in the abbey are responsible for this (DRAMATIC SETTING) striking (DRAMATIC), on site (SETTING) (DRAMATIC SETTING),
18 Tweets (MESSAGES) from wise guy (-SAGE-) in disarray (MES-S) (MESSAGES),
20 Half the local food (local ‘food’ = grub) (half of ‘GRub’ = GR-) is not tough (-EASY) (GREASY) or oily (GREASY),
22 Demoralise (DISHEARTEN) with the sardine (the sardine) dish (= anagram indicator) (the sardine = DISHEARTEN),
24 A danger to shipping (REEF) in Lough (Lough (REE -) Foyle from the start (‘Foyle’ from the start = -F) (REEF)
25 Dance (ball = B- -ALL) welcomes jazz (= anagram indicator) here (here = -EER H-) (BEER HALL) in Munich drinking establishment (BEER HALL),
26 The trademark image (EMBLEM) of upsetting (= reversal indicator) viral phenomenon (meme = EM-EM) about empty (a) laboratory (lab) (‘lab’ without ‘a’ = lb = -BL-) (EMBLEM).
Down:
1 Plays (DRAMAS) down in the dumps (sad = D-AS) around ruin (mar = -RAM-), in retrospect (= reverse indicator) (DRAMAS),
2 Polish prose (EDIT) at the heart of 16 down? (heart of ‘reEDITed’ = EDIT),
3 First course (first ‘Course’ = C-) at public house (bar) is sent back (= reverse indicator) (bar = -RAB-) – it’s not for a vegetarian (-MEAT) (CRABMEAT) or for a pescatarian to eat (CRABMEAT),
4 Revolutionary (= reverse indicator) characters in Othello discouraging (characters in ‘othelLO, DIscouraging’ = lodi = IDOL) celebrity (IDOL),
5 Tried (HAD A GO) to halt first (‘Halt’ first = H-) bill (-AD-) by government lawyer (Attorney General = -A G-) and it came to nothing (-O) (HAD A GO),
6 Friend (mate = MA-TE) overlooks river in Spain (Spanish ‘river’ = -RIO-) with piece of angling equipment (-NET-) (MARIONETTE) – the type that comes with strings attached (MARIONETTE),
7 Break the law (SIN-) joining National Front (-N F-) from a German (‘a’ German = -EIN) (SINN FEIN) party (SINN FEIN),
13 Undertone (ATMOSPHERE) to a (A-) most (most) bizarre (= anagram indicator) (most = -TMOS-) post (-P-) at this point (-HERE) (ATMOSPHERE),
14 Local expresses derision (local ‘derision’ = SUCKS) with Pole (South = S-) losing the head (d) with Huey, Dewey and Louie (three nephews of Donald Duck = ducks) (‘ducks’ without ‘d’ = -UCKS) (SUCKS),
16 Grass (reed = REE-D) around old institute (Dublin Institute of Technology = -DIT-) on point (compass point East = -E-) (RE-EDITED) is cut again (RE-EDITED),
17 Greene in (Greene in) adapting (= anagram indicator) (Greene in = ENGINEER) plot? (ENGINEER),
19 Leave (GO AWAY) Indian State (GO A-) by road (-WAY) (GO AWAY),
21 Starter of haddock (starter of ‘Haddock’ = -H-) covered in leeks (leeks) baked (= anagram indicator) (leeks = S-EKEL) (SHEKEL) with dough (money) from Haifa? (SHEKEL),
23 Some costal kibbutz (found in ‘cosTAL Kibbutz’ = TALK) gossip (TALK),
24 Covered up (= reverse indicator) by Cesare Borgia’s (covered by ‘cesarE BORgia’s’ = ebor = ROBE) houppelande, for instance (houppelande, for instance/ 15th century ROBE).