Ease off with rum in music bar (7)
I believe the answer is:
measure
'music bar' is the definition.
(measure can mean a bar of music)
'ease off with rum' is the wordplay.
'off with' indicates an anagram.
'ease' anagrammed with 'rum' gives 'MEASURE'.
'in' acts as a link.
(Other definitions for measure that I've seen before include "See 1" , "Ascertain the length etc." , "Ascertain the size of" , "Establish the size" , "Portion" .)