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' (cur is a kind of dog).
'taking' is a charade indicator (letters next to each other).
'heartless' suggests removing the centre.
'bites' with its middle removed is 'bs'.
'cur'+'bs'='CURBS'
(Other definitions for curbs that I've seen before include "Keeps in check" , "Restrains" , "Place restrictions on" .)