Joints leftwing uncle proposed, initially, to hold 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')
'joints leftwing uncle proposed initially to hold' is the wordplay.
I cannot really understand how this works, but
'joints' could be 'knees' (knee is a kind of joint) and 'knees' is found in the answer.
The remaining letters 'up' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for knees-up that I've seen before include "Informal party with dancing" , "Dance with Mother Brown" , "Informal shindig (with Mother Brown?)" , "Lively party (colloq.)" , "Riotous dance - party" .)