Set up bar for party (5)
I believe the answer is:
revel
'for party' is the definition.
(I've seen this before)
'set up bar' is the wordplay.
'set up' is a reversal indicator (in a down clue, letters go up).
'bar' becomes 'lever' (lever is a kind of bar).
'lever' in reverse letter order is 'REVEL'.
(Other definitions for revel that I've seen before include "Carouse" , "Spree" , "Rejoice" , "Have a few drinks etc." , "Celebrate noisily" .)