File about registered inmate (5)
I believe the answer is:
lifer
'inmate' is the definition.
(lifer is a kind of inmate)
'file about registered' is the wordplay.
'about' is an anagram indicator.
'registered' becomes 'r' (abbreviation, as in registered trademark).
'file' with letters rearranged gives 'life'.
'life'+'r'='LIFER'
(Other definitions for lifer that I've seen before include "Prisoner serving the maximum term" , "Con" , "One with maximum sentence" , "One with very long sentence" , "Serious criminal" .)