Old mice turned out to be musical (7)
I believe the answer is:
melodic
'musical' is the definition.
(thesaurus)
'old mice turned out' is the wordplay.
'turned out' indicates an anagram.
'old'+'mice'='oldmice'
'oldmice' is an anagram of 'MELODIC'.
'to be' is the link.
(Other definitions for melodic that I've seen before include "Pleasant to listen to" , "Of music, tuneful and sweet" , "Pleasant-sound" , "Old mice (anag.)" , "Harmonious" .)