Time spanned by piece of music fine for composer (6)
I believe the answer is:
bartok
'for composer' is the definition.
(20th-century Hungarian composer Bela Bartok)
'time spanned by piece of music fine' is the wordplay.
'time' becomes 't' (abbreviation).
'spanned by' indicates putting letters inside.
'piece of music' becomes 'bar' (bar of music).
'fine' becomes 'OK' (synonyms).
'bar'+'ok'='barok'
't' inserted into 'barok' is 'BARTOK'.
(Other definitions for bartok that I've seen before include "Bela -, nineteenth and twentieth century Hungarian composer" , "Hungarian composer and pianist, d. 1945" , "classical composer" , "Bela -, Hung. composer" , "Bela ------ (1881-1945), Hungarian composer" .)