Make merry back in bar (5)
I believe the answer is:
lever
'bar' is the definition.
(I know that lever is a type of bar)
'make merry back' is the wordplay.
'make merry' becomes 'revel' (synonyms).
'back' is a reversal indicator.
'revel' reversed gives 'LEVER'.
'in' acts as a link.
(Other definitions for lever that I've seen before include "Revel with prising tool" , "Bar turning on a fulcrum" , "Purchase from bar" , "Rigid bar pivoting about a fulcrum" , "Exert pressure" .)