Part of leg, mine, in knees-up (7)
I believe the answer is:
shindig
'knees-up' is the definition.
(I've seen this before)
'part of leg mine' is the wordplay.
'part of leg' becomes 'shin' ('shin' is part of 'leg').
'mine' becomes 'dig' (I've seen this before).
'shin'+'dig'='SHINDIG'
'in' is the link.
(Other definitions for shindig that I've seen before include "Noisy knees-up" , "Hidings (anag) - party" , "Large, lively party" , "Noisy disturbance" , "Bash" .)