ACROSS
 1. PARA AIRPLANE* outside H = PARAPHERNALIA <equipment>
 9. _A_B_S _U_R_D_I_ S_M_ = ABSURDISM <existential philosophy>
10. "COPS" = COPSE <wooded area>
11 <aim> TARGET = TAR GE + T
12. <party animal> REVELLER = REVEILLE R - I
14. RIG H TANGLE = RIGHT ANGLE <5400 minutes?>
16. <provide free> COMP <state school>
18. <European> POLE = "POLL"
19. (YANKEE IS)* N_ S_ = KEYNESIANS <economic theorists>
22. <inimitable> PEERLESS = PEERESS outside L
23. DANG + _ER_ = DANGER <peril>
26. E_ VENT = EVENT <tournament, perhaps>
27. A WEST + RUCK = AWESTRUCK <overcome by wonderment>
28. EX PRES + STRAINS = EXPRESS TRAINS <they're fast-tracked?>

DOWN
 1. P_ + LATTER = PLATTER <dish>
 2. <part of flight> RISER = _R _I _S _E _R
 3. <infiltrate> PERMEATE = PER TEAM* outside E
 4. ←TIME← = EMIT <issue>
 5. (.g)NUM BERLIN (e.) = NUMBER LINE <some negatives then some positives>
 6. LA C_ + KEY = LACKEY <henchperson>
 7. A PP ALOO'S + A = APPALOOSA <horse>
 8. ⭯TRIPLE⭯ = LET RIP <show no restraint>
13. <cross-country runner> INTERSTATE = INTESTATE outside R
15. "GOAL DENY" = GOLDENEYE <satellite weapon from spy film>
17. <check condition of> ASK AFTER = (ART FAKES)*
18. POP + PET = POPPET <child>
20. SS outside TRIKE = STRIKES <doesn't work>
21. FL ←PITY← = FLY-TIP <to illegally dump trash>
24. <architect> GAUDI = GAUD + I_
25. <witnesses> SEES = "SEIZE"