Joint drink party (5-2)
I believe the answer is:
knees-up
'party' is the definition.
(I have seen 'Lively party' mean 'knees up' so perhaps 'party' could also mean 'knees up')
'joint drink' is the wordplay.
'joint' becomes 'knee' (knee joint is a kind of joint).
'drink' becomes 'sup' (to sup is to drink).
'knee'+'sup'='KNEES-UP'
(Other definitions for knees-up that I've seen before include "Riotous dance - party" , "Informal party with dancing" , "Lively informal party" , "Dance with Mother Brown" , "Lively party (colloq.)" .)