Composer’s brother, one released from prison (6)
I believe the answer is:
franck
'composer's' is the definition.
(Cesar Franck)
'brother one released from prison' is the wordplay.
'brother' becomes 'fra' (title for an Italian monk).
'one' becomes 'i' (Roman numeral).
'released from' is a deletion indicator.
'prison' becomes 'nick' (slang term for prison).
'nick' with 'i' taken away is 'nck'.
'fra'+'nck'='FRANCK'
(Other definitions for franck that I've seen before include "Cesar -, Belg. composer; rhymes with cheap wine" , "Cesar ---, Fr. composer" , "Cesar ___ , Belgian-born French composer, d. 1890" , "Cesar --, Belgian composer" , "He scored" .)