Muzzle small bear that's devoured rook (4)
I believe the answer is:
curb
'muzzle' is the definition.
(both can mean to restrain or suppress)
'small bear that's devoured rook' is the wordplay.
'small bear' becomes 'cub' (a young bear).
'that's devoured' is an insertion indicator (inserted letters get devoured).
'rook' becomes 'R' (chess abbreviation).
'cub' going around 'r' is 'CURB'.
(Other definitions for curb that I've seen before include "Strap used to restrain horse" , "Hold back or try to prevent" , "Restrain, inhibit" , "'Temper, hold in restraint (4)'" , "Constrain" .)