Prisoner's only comfort (7)
I believe the answer is:
console
'comfort' is the definition.
(synonyms)
'prisoner's only' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'only' becomes 'sole' (synonyms).
'con'+'sole'='CONSOLE'
(Other definitions for console that I've seen before include "give reassurance" , "Comfort - control unit" , "where the organist sits" , "Comfort - cabinet" , "Key-desk of an organ" .)