Gun is out of order in practice (5)
I believe the answer is:
using
'practice' is the definition.
(I know that using is a type of mistreatment)
'gun is out of order' is the wordplay.
'out of order' indicates anagramming the letters.
'gun'+'is'='gunis'
'gunis' anagrammed gives 'USING'.
'in' is the link.
(Other definitions for using that I've seen before include "Exercising" , "Putting into service" , "Exploiting for one's own advantage" , "taking drugs" , "Utilising" .)