Prisoner’s popular with friend (6)
I believe the answer is:
inmate
'prisoner's' is the definition.
(inmate is a kind of prisoner)
'popular with friend' is the wordplay.
'popular' becomes 'in'.
'with' is a charade indicator (letters next to each other).
'friend' becomes 'mate' (mate is a kind of friend).
'in'+'mate'='INMATE'
(Other definitions for inmate that I've seen before include "Person serving a jail sentence" , "Lag" , "One in prison/hospital" , "One is in jail" , "Inhabitant of prison or other institution" .)