Celebrate useful bar being set up (5)
I believe the answer is:
revel
'celebrate' is the definition.
(I know that revel is a more specific form of the action celebrate)
'useful bar being set up' is the wordplay.
'useful bar' becomes 'lever' (a lever is bar with some purpose or function).
'being set up' is a reversal indicator (in a down clue, letters go up).
'lever' reversed gives 'REVEL'.
(Other definitions for revel that I've seen before include "Celebrate with a lever, strangely" , "Carouse" , "Rejoice" , "Take great pleasure" , "Take great delight (in)" .)