Restrict prisoner very well (7)
I believe the answer is:
confine
'restrict' is the definition.
('confine' can be a synonym of 'restrict')
'prisoner very well' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'very well' becomes 'fine' ('fine' can be a synonym of 'very well').
'con'+'fine'='CONFINE'
(Other definitions for confine that I've seen before include "pen" , "Restrict (to an area)" , "Incarcerate" , "Keep within limits" , "Shut in" .)