Prisoner to make fit in (7)
I believe the answer is:
conform
'fit in' is the definition.
(thesaurus)
'prisoner to make' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'to make' becomes 'form' ('form' can be a synonym of 'make').
'con'+'form'='CONFORM'
(Other definitions for conform that I've seen before include "Toe the line" , "Fall into line" , "Comply with accepted rules" , "Match" , "Keep to rules or general custom" .)