Availing oneself of gun is wrong (5)
I believe the answer is:
'availing oneself of' is the definition.
(I've seen this in another clue)
'gun is wrong' is the wordplay.
'wrong' indicates anagramming the letters.
'gunis' with letters rearranged gives 'USING'.
(Other definitions for using that I've seen before include "Exercising" , "having recourse to" , "addiction" , "practice" , "taking drugs" .)