Serious criminal misused rifle (5)
I believe the answer is:
lifer
'serious criminal' is the definition.
(someone serving a life sentence in prison)
'misused rifle' is the wordplay.
'misused' indicates an anagram.
'rifle' is an anagram of 'LIFER'.
(Other definitions for lifer that I've seen before include "convict given longest sentence" , "One with maximum sentence" , "Convicted murderer, say" , "Rifle (anag.)" , "Person serving long sentence" .)