Popular drink for prisoner? (6)
I believe the answer is:
inmate
'for prisoner?' is the definition.
(inmate can mean someone in prison)
'popular drink' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'drink' becomes 'mate' (mate is a kind of drink).
'in'+'mate'='INMATE'
(Other definitions for inmate that I've seen before include "Someone confined, eg to hospital or prison" , "Person serving a jail sentence" , "Person confined to institution" , "One is in jail" , "Inhabitant of prison or other institution" .)