Social gathering’s ale unfinished (3)
I believe the answer is:
bee
'social gathering's' is the definition.
(bee is a kind of social gathering)
'ale unfinished' is the wordplay.
'ale' becomes 'beer' (I've seen this before).
'unfinished' means to remove the last letter.
'beer' with its last letter removed is 'BEE'.
(Other definitions for bee that I've seen before include "Useful insect" , "A honey-maker" , "competition" , "Hive insect" , "Creator of honey" .)