Popular friend, prisoner (6)
I believe the answer is:
inmate
'prisoner' is the definition.
(inmate is a kind of prisoner)
'popular friend' is the wordplay.
'popular' becomes 'in'.
'friend' becomes 'mate' (mate is a kind of friend).
'in'+'mate'='INMATE'
(Other definitions for inmate that I've seen before include "Someone confined, eg to hospital or prison" , "Occupant" , "One living in (prison)" , "Cell resident" , "eg Prisoner" .)