Prisoner to look after, say (7)
I believe the answer is:
contend
'say' is the definition.
(thesaurus)
'prisoner to look after' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'to look after' becomes 'tend' (tending is a kind of looking after).
'con'+'tend'='CONTEND'
(Other definitions for contend that I've seen before include "Struggle; assert" , "Tend NCO (anag.)" , "fight" , "Struggle in rivalry" , "Struggle (with someone)" .)