ACROSS
 1. <divulge> IMPART = ←MI← PA + RT
 4. <audacity> CHUTZPAH = CH UTAH outside Z_ P_
10. (PERILOUS DESTINY)* = SERENDIPITOUSLY <by a conveniently-timed stroke of luck>
11. <make small> DIMINISH = DISH outside I'M IN
12. (.t)ALK ALI(e.) = ALKALI <base>
14. <domineering> ROUGHSHOD ⇒ SHOD* {dosh}
16. "PEAK" = PIQUE <displeasure>
18. J__O__T_ _U__N = JOTUN <enemy of the gods?>
19. <flawless> EXEMPLARY = (MARY outside PL) after EX _E
21. "COCK SIX" = COCCYX <a bit of backbone>
23. RECITE* outside SO = ESOTERIC <hard to understand>
26. <reporter> PHOTOJOURNALIST = P HOT [-s]OJOURN + A-LIST
27. EX TB inside TOOK = TEXTBOOK <19 {exemplary}>
28. DAMN outside SO = DAMSON <fruit>

DOWN
 1. <member of staff> INSIDER = (_INIDRES_)*
 2. <leading> PARAMOUNT = PARA + MOUNT
 3. R + ANON = RAN ON <kept talking and talking>
 5. <shock> HAIR = H AIR
 6. ←PIPE LOOT outside D← = TOODLE-PIP <so long>
 7. PA'S + T_ + A = PASTA <fare>
 8. <out-of-control> HAYWIRE = →"WAY HIGHER"←
 9. (IS THICKER)* = KITSCHIER <less sophisticated>
13. <opponent> ADVERSARY = ADVER[-b] + SA + R_Y
15. (MY COB)* after HONE = HONEYCOMB <layer of sweetness>
17. QUA_ + TRAINS = QUATRAINS <groups of four lines>
18. JACK POT = JACKPOT <kitty>
20. CAT inside YUAN = YUCATAN <peninsula>
22. "SHOO" = CHOUX <dough>
24. TA outside (I + AR) = TIARA <headpiece>
25. MOO[-n] outside J = MOJO <magical power>