Unmarried composer got part back (8)
I believe the answer is:
bachelor
'unmarried' is the definition.
(thesaurus)
'composer got part back' is the wordplay.
'composer' becomes 'Bach' (Johann Sebastian Bach).
'got' means one lot of letters go next to another.
'part' becomes 'role' (eg a theatrical role).
'back' says the letters should be written backwards.
'role' backwards is 'elor'.
'bach'+'elor'='BACHELOR'
(Other definitions for bachelor that I've seen before include "Single person" , "Unmarried male" , "he has no ties" , "Non-married male" , "Unmarried man (with a degree?)" .)