One may have used rifle criminally? (5)
I believe the answer is:
lifer
'one may have' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'rifle criminally?' is the wordplay.
'criminally?' indicates an anagram (I've seen 'criminally' mean this (I've seen this in other clues)).
'rifle' with letters rearranged gives 'LIFER'.
'used' is the link.
(Other definitions for lifer that I've seen before include "Convicted murderer, say" , "Con" , "One behind bars for ever" , "Long-term prisoner" , "convict given longest sentence" .)