Treating villain admitting offence (6)
I believe the answer is:
dosing
'treating' is the definition.
(dosing is a kind of treating)
'villain admitting offence' is the wordplay.
'villain' becomes 'dog' (dog is a kind of villain).
'admitting' is an insertion indicator.
'offence' becomes 'sin' (I've seen this before).
'dog' enclosing 'sin' is 'DOSING'.
(Other definitions for dosing that I've seen before include "Giving medicine" , "Dispensing" .)