Pig joins crowd hosting a winter celebration (8)
I believe the answer is:
hogmanay
'winter celebration' is the definition.
'pig joins crowd hosting a' is the wordplay.
'pig' becomes 'hog' (synonyms).
'joins' is a charade indicator (letters next to each other).
'crowd' becomes 'many' (a crowd is made up of many people).
'hosting' is an insertion indicator (inserted letters are welcomed in or hosted).
'many' placed around 'a' is 'manay'.
'hog'+'manay'='HOGMANAY'
(Other definitions for hogmanay that I've seen before include "New Year's Eve in Scotland" , "Festival" , "Scottish New Year's Eve" , "Scottish celebration framed in mahogany" , "New Year's Eve north of the border" .)