Prisoner's fashionable friend (6)
I believe the answer is:
inmate
'prisoner's' is the definition.
(inmate can mean someone in prison)
'fashionable friend' is the wordplay.
'fashionable' becomes 'in' (in can mean fashionable or trendy).
'friend' becomes 'mate' (mate is a kind of friend).
'in'+'mate'='INMATE'
(Other definitions for inmate that I've seen before include "Inhabitant of prison or hospital" , "Inhabitant of prison or other institution" , "Cell resident" , "Convict" , "eg Prisoner" .)