Celebration in bar over (5)
I believe the answer is:
revel
'celebration' is the definition.
(I know that revel is a type of conviviality)
'bar over' is the wordplay.
'bar' becomes 'lever' (lever is a kind of bar).
'over' says the letters should be written backwards.
'lever' reversed gives 'REVEL'.
'in' acts as a link.
(Other definitions for revel that I've seen before include "Lever (anag.)" , "Be festive, enjoy" , "Be festive, have a good time" , "Paint the town red" , "Spree" .)