Make merry in bar, falling over (5)
I believe the answer is:
revel
'make merry' is the definition.
(synonyms)
'bar falling over' is the wordplay.
'bar' becomes 'lever' (lever is a kind of bar).
'falling over' says the letters should be written backwards (letters falling back).
'lever' back-to-front is 'REVEL'.
'in' acts as a link.
(Other definitions for revel that I've seen before include "Lever (anag.)" , "Celebrate noisily" , "Be festive, enjoy" , "Paint the town red" , "Riotous feast" .)