Emus are let loose in music bar (7)
I believe the answer is:
measure
'music bar' is the definition.
(measure can mean a bar of music)
'emus are let loose' is the wordplay.
'let loose' is an anagram indicator.
'emus'+'are'='emusare'
'emusare' is an anagram of 'MEASURE'.
'in' is the link.
(Other definitions for measure that I've seen before include "Act - dance" , "of specified size" , "Determine the dimensions" , "bar of music" , "See 6" .)