Muzzle dog with boxer's lead (4)
I believe the answer is:
curb
'muzzle' is the definition.
(both can mean to restrain or suppress)
'dog with boxer's lead' is the wordplay.
'dog' becomes 'cur' (both can mean a despicable person).
'with' is a charade indicator (letters next to each other).
'lead' suggests taking the first letters.
The initial letter of 'boxers' is 'b'.
'cur'+'b'='CURB'
(Other definitions for curb that I've seen before include "Constrain" , "Hold back or try to prevent" , "[LIMITATION]" , "Control, limit" , "Check or restrain" .)