Restrains a dog taking heartless bites (5)
I believe the answer is:
curbs
'restrains' is the definition.
(curbing is a kind of restraining)
'a dog taking heartless bites' is the wordplay.
'a dog' becomes 'cur' (a cur is an aggressive or scruffy dog).
'taking' means one lot of letters go next to another.
'heartless' means to remove the middle letters.
'bites' with its centre removed is 'bs'.
'cur'+'bs'='CURBS'
(Other definitions for curbs that I've seen before include "Keeps in check" , "Restrains" , "Place restrictions on" .)