Gun is dismantled for practice (5)
I believe the answer is:
using
'practice' is the definition.
(using is a kind of practice)
'gun is dismantled' is the wordplay.
'dismantled' indicates anagramming the letters.
'gun'+'is'='gunis'
'gunis' is an anagram of 'USING'.
'for' acts as a link.
(Other definitions for using that I've seen before include "Exploiting for one's own advantage" , "taking drugs" , "Utilising" , "practice" , "addiction" .)