ACROSS
1. C ROW + NPR INC E = CROWN PRINCE <next head of state>
7. <one getting baked> PIE = PI E
9. <kingdom> REALM = (-since)RE ALM(-sgiving)
10. STOPPED* outside ON = POSTPONED <sort of stopped keeping on?>
11. <small plant> SEEDLING = LEEDS* + I N + G
12. <take control of> CORNER = CO RN + E R
14. M ALT = MALT <type of whisky>
16. "PALE" = PAIL <bucket>
17. THUMB ¿?digital [pertaining to fingers, thumbs and toes] opposition [thing that opposes, as opposable thumbs do]¿?
20. ←TARPS← = SPRAT <fish>
21. "CAUSE" = CAWS <bird noises>
22. TAR (-c)A(-n) = TA-RA <see you>
24. G (L inside LADY) = GLADLY <without complaint>
25. <pickpockets> TROUSERS = TRO USERS
28. <all the more reason> A FORTIORI = (FOR RIOT*) inside AI
29. PAN ←AD← = PANDA <endangered mammal>
30. E leaves TOTE = TOT <child>
31. CAN D (LEST I) C(-oo)K = CANDLESTICK <somewhere to put heat source>

DOWN
1. <holiday> CHRISTMAS = ON leaves MONARCHISTS*
2. <speak> ORATE = O RATE
3. <swift> NIMBLE = NILE outside MB
4. REP + ENT(-r)ANCE = REPENTANCE <wanting to make things better>
5. SENT* = NEST <to brood>
6. XP inside (I STOLE)* = EXPLOITS <heroic accomplishments>
7. ←(-tant)alus' nine p(-unishments)← = <intermediate place between mainland and island?>
8. <duck> EIDER = EID + ER
13. FLO ←GREW← IRL = FLOWER GIRL <street trader>
15. L + ARK AB OUT = LARK ABOUT <to have some fun>
18. <green climber> BEANSTALK = BEANS + TALK
19. (HIT CLEAT)* = ATHLETIC <suitable for sports>
23. <chaos> RUMPUS = RUMP US
24. <massive> GIANT = GIT outside AN
26. <feeling of boredom> ENNUI = "ON WII"
27. <side with> JOIN = J + (-d)O IN

THEME: JACK

- jack be NIMBLE, jack be quick, jack jump over the CANDLESTICK
- little jack horner, sat in his CORNER, eating his CHRISTMAS PIE. he stuck out his THUMB...
- jack and jill went up the hill to fetch a PAIL of water. jack fell down and broke his CROWN...
- jack and the BEANSTALK/jack the GIANT-slayer
- jack SPRAT