Keep prisoner sunny (7)
I believe the answer is:
confine
'keep' is the definition.
(I know that keep is a more specific form of the action confine)
'prisoner sunny' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'sunny' becomes 'fine' (as in fine weather).
'con'+'fine'='CONFINE'
(Other definitions for confine that I've seen before include "pen" , "Shut in" , "Restrict (to an area)" , "Keep within limits" , "Imprison" .)