Facing heartless canine (4)
I believe the answer is:
fang
'canine' is the definition.
(I've seen this before)
'facing heartless' is the wordplay.
'heartless' means to remove the middle letters.
'facing' with its centre removed is 'FANG'.
(Other definitions for fang that I've seen before include "Dracula's tooth?" , "possible poisoner" , "Animal's sharp tooth" , "Large canine tooth" , "Poisonous tooth" .)