Eg prisoner’s trendy sidekick (6)
I believe the answer is:
inmate
'eg prisoner's' is the definition.
(I've seen this before)
'trendy sidekick' is the wordplay.
'trendy' becomes 'in' ('in' can mean fashionable).
'sidekick' becomes 'mate' (sidekick can mean a friend or ally).
'in'+'mate'='INMATE'
(Other definitions for inmate that I've seen before include "One held in prison" , "Person serving a jail sentence" , "Con" , "One institutionally confined" , "Institution resident" .)