ACROSS
 1. V CHILE* + E = VEHICLE <car?>
 5. <arm support> HUMERUS = HUM + ←SURE←
 9. <one splitting up> SEPARATOR = (OPERA STAR)*
10. <number> SEVEN = (IS - I) EVEN
11. <hormone> NOREPINEPHRINE = NO RE (PINE PINE) outside HR
14. <group of players> TEAM = "TEEM"
15. <big city> MANCHESTER = MERCHANTS* outside _E
18. <practice> EXPERIENCE = EX PENCE outside RI _E
19. _A_R_E_ + A = AREA <field>
21. (I READ NOTING ITS)* = DISINTEGRATION <sudden lack of cohesion>
24. H inside NICE = NICHE <nook>
26. <wipe out> ERADICATE = C_ inside E RADIATE
27. BAs inside EMs + _Y = EMBASSY <diplomatic building>
28. "HIGH JEAN" = HYGIENE <cleanliness>

DOWN
 1. <huge> VAST = VAs _T
 2. <learning centre> HIPPOCAMPUS = HIP ←OP← + CAMPUS
 3. _ONCERT_* = CORNET <brass instrument>
 4. EXTRA T_E outside I_ C = EXTRICATE <free>
 5. "HOARSE" = HORSE <racing animal>
 6. <skulduggery> MISCHIEF = MI's CHIEF
 7. <gun {e.g. a car engine}> REV = (.r)REV(o.)
 8. <tongue maybe> SENSE ORGAN = (NOSE'S RANGE)*
12. <question> INTERROGATE = (TREATING ORE)*
13. <turnout> ATTENDANCE = (AT TEN) DANCE
16. →"ROCK COACH"← = COCKROACH <pest>
17. PR (ANT out of INCESSANT) = PRINCESS <royal daughter>
20. <list of characters> STRING = R inside STING
22. <funeral lament> ELEGY = EG inside ELY
23. (ME ME) = MEME <viral Internet content>
24. <red {wine}> CAB <taxi>