7. MERRY
Cheerful performer Rylance included (5)

Hidden answer – perforMER RYlance, indicated by ’included’ – Actor Mark Rylance usually has a rather sombre demeanour. 

This, of course, is the one solution that must go before…

8. CHRISTMAS
Holiday season cast its charms (9)

Anagram, indicated by ‘cast’ (thrown) of ‘ITS CHARMS’. 

10. JUMPER
Worn out in cold spring before emergency room (6)

Spring = JUMP before Emergency Room, abbreviated to ER.

First of the solutions to follow 8 across, though the fashion for ‘hilarious’ Christmas jumpers may not be quite what it was. 

11. STOCKING
Leg wear filling shelves (8)

Double definition. The leg wear meaning makes it the second linked solution.

12. NUTSHELL
Maybe strewn across table after festive meal – crazy nightmare! (8)

Crazy = NUTS. Nightmare = HELL. 

13 CARD
Dracula comes back, hides ace (4)

Hidden answer, indicated by ‘hides’ this one reversed, indicated by ‘comes back’. DRACula. Third of the linked solutions.

15. GLASSES
Good girls see drinks in these (7)

Good = G. Girls = LASSES. Also, they might be more likely to see their drinks if they wore their glasses!

17. CRACKER
Biscuit goes snap! (7)

Double definition. Again the one that goes snap comes after 8. 

20. CAKE
Baked treat, crackles oddly (4)

Answer hidden in alternate letters of CrAcKlEs indicated by ‘oddly’. 

Another linked solution. 

22. RED ALERT
Excited adulterers, not us, in urgent state of readiness (3,5)

Anagram, indicated by ‘excited’ of ‘ADuLTERERs’ excluding the U and S (not us)

25. BARRACKS.
Pours scorn on pub storage (8)

Pub = BAR Storage = RACKS.

27. DINNER.
Left winger upset about hotel meal (6)

Left winger = red, reversed (upset) to D ER about (around) Hotel = INN. 

Linked to 8. 

28. BOYFRIEND
Lively robin fed round yard by young man (9)

Anagram, indicated by ‘lively’ of ‘ROBIN FED’ round Yard = Y. 

29. PARTY
Quiet, creative gathering (5)

Quiet = P (musical notation) Creative = ARTY

Another linked solution.

1. TENUOUSLY
Lousy tune swirling thinly (9)

Anagram (swirling) of ‘LOUSY TUNE’.

2. PROPOSES
Suggests supporting models (8)

Supporting = PRO. Models = POSES

3. WHISTLE
Shrill sound of boundless glee after game (7)

gLEe missing beginning and end (boundless) after Game = WHIST. 

4. LIDO
Cover nothing in pool (4) 

Cover = LID. Nothing = O

5. STOKER
Potteries regional leader adds fuel to fire (6)

Potteries = STOKE (City at the heart of the Staffordshire potteries area) regional leader (first letter of Regional)

6.YAWNS
Way back? Contradictory directions happen when up early (or late) (5)

 ‘Way’ reversed (back) to YAW. N = North and S = South (Contradictory directions)

9. TREE
Trio, missing husband, star supporter (4)

Trio = ThREE missing h for husband. Definition applies to Christmas trees but not all trees I concede, and only those not supporting an Angel I suppose!

Another linked solution. 

13. CAR
Motoring club reverses vehicle (3)

Royal Automobile Club = RAC  reversed.

14. REORIENTS
Retires on moving, faces east again (9)

Anagram (Moving) of ‘RETIRES ON’.

16.EVE
Girl seen the night before? (3)

Double definition. The ? acknowledges the definition by example. 

18.CALENDAR
Cradle an arrangement maybe, for Advent (8)

Anagram, Indicated by ‘arrangement’ of ‘CRADLE AN’.

19.PRESENT
Now in attendance under 9 (7)

Double definition. Though it’s the sort of present found under a tree (9) that makes this another linked solution.

21. ADRIFT
Off course in first snowbank (6)

First = A, as in A team = first team. Snowbank = DRIFT.

23. DUDE
Guy starts to drink until dancing ends (4)

Initial letters (starts to) of Drink Until Dancing Ends.

24. CAROL
Does she make seasonal music? (5)

Double definition and definition by example again as with EVE above. 

26. CRIB
Copy nativity scene (4)

Double definition and the final linked solution.