ACROSS
 1. DISCOVERIES* outside A_ D_ = DO A DISSERVICE <cause harm>
10. <best> ONE-UP = OUP outside N E_
11. CHANCE, LEAR←_H = CLEARANCE <blessing>
12. <&lit> TABLETOP = TABLET + OP
13. <impulsive> MADCAP = MAP outside A DC
15. C_Y A_N = CYAN <blue>
16. <connect electronics> DAISY CHAIN = DAIS + YIN outside CHA
19. <write too little music> UNDERSCORE <line for emphasis?>
20. <car> LIMO = L IMO
22. T (R inside HUSH) = THRUSH <singer>
24. <hostile> INIMICAL = (I_ CLAIMIN_)*
26. CUE INFORM, N← = CUNEIFORM <ancient writing system>
27. "UTE EARN" = U-TURN <manoeuvre>
28. LO + NES + TARS + TATE = LONE STAR STATE <Texas>

DOWN
 2. VERB inside O-O _A_ RD = OVERBOARD <too much>
 3. <well-dressed> DAPPER = ←REP + PA D_←
 4. (CO-STARS CAN)* = SACROSANCT <beyond criticism>
 5. "USE" = EWES <members of the flock>
 6. V AMERICAN* - M_ = VARIANCE <diversity>
 7. (.n)CY NIC(k.) = CYNIC <world-weary sort?>
 8. POE TIC JUST + ICE = POETIC JUSTICE <karma>
 9. BOWL inside TEN PINING = TENPIN BOWLING <skittles>
14. (ROOSTER MAN)* = ASTRONOMER <Celsius perhaps>
17. <battle> AGINCOURT = A GIN + COURT
18. <perfect> PRISTINE = TIN inside PRISE
21. <measure> AMOUNT = A MOUNT
23. <complete stranger> RANDO = (R_ AND _O)
25. <plug> TOUT = _T OUT