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