Prisoner's popular partner (6)
I believe the answer is:
inmate
'prisoner's' is the definition.
(inmate is a kind of prisoner)
'popular partner' is the wordplay.
'popular' becomes 'in'.
'partner' becomes 'mate' ('mate' can be a synonym of 'partner').
'in'+'mate'='INMATE'
(Other definitions for inmate that I've seen before include "One living in institution" , "Lag" , "you mightn't want him to escape" , "Inhabitant of prison or other institution" , "Person serving a jail sentence" .)