Convict fashionable couple (6)
I believe the answer is:
Here is my best explanation:
'convict' is the definition.
(I've seen this before)
'fashionable couple' is the wordplay.
'fashionable' becomes 'in' ('in' can be similar in meaning to 'fashionable').
'couple' becomes 'mate' ('mate' can be a synonym of 'couple'**).
(Other definitions for inmate that I've seen before include "One is confined to an institution", "Someone confined, eg to hospital or prison", "you mightn't want him to escape", "One institutionally confined", "One is in jail", "Con", "Institution resident", "Lag", "con" and "Person serving a jail sentence".)