Part of leg, mine, in knees-up (7)
I believe the answer is:
Here is my best explanation:
'knees-up' is the definition.
'part of leg mine' is the subsidiary indication.
'part of leg' becomes 'shin' ('shin' is part of 'leg').
'mine' becomes 'dig' (I've seen this in other clues).
'in' is the link.
(Other definitions for shindig that I've seen before include "Commotion", "Noisy disturbance", "Party", "A knees-up", "Hooley", "Large, lively party", "Large and noisy party or quarrel", "Lively party, knees-up", "Rumpus" and "Hidings (anag) - party".)