Restraint in bridle as heavy? (5)
I believe the answer is:
leash
'restraint' is the definition.
(leash is a kind of restraint)
'in bridle as heavy?' is the wordplay.
'in' indicates a hidden word.
'LEASH' is hidden within 'bridle as heavy'.
(Other definitions for leash that I've seen before include "Line for holding dog" , "A dog walker may use it" , "Strap for holding a dog while walking" , "Strip for leading, a dog say" , "Animal restraint" .)