Existence right for inmate (5)
I believe the answer is:
lifer
'for inmate' is the definition.
(lifer is a kind of inmate)
'existence right' is the wordplay.
'existence' becomes 'life' (being alive).
'right' becomes 'r' (common abbreviation).
'life'+'r'='LIFER'
(Other definitions for lifer that I've seen before include "Person serving long sentence" , "convict given longest sentence" , "person given sizeable bird" , "One given lots of porridge" , "Long-term prisoner" .)