Knees-up (7)
I believe the answer is:
shindig
'knees-up' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for shindig that I've seen before include "Lively party, knees-up" , "Large, lively party" , "Bash" , "Big lively party (colloq.)" , "wild party" .)