Have no mercy, harbouring such spitefulness (5)
I believe the answer is:
venom
'spitefulness' is the definition.
('venom' can be a synonym of 'spitefulness')
'have no mercy harbouring' is the wordplay.
'harbouring' indicates a hidden word.
'VENOM' can be found hidden inside 'have no mercy'.
'such' is the link.
(Other definitions for venom that I've seen before include "Poison; malice" , "Snake's poison" , "acrimony" , "ill will" , "Poisonous secretion" .)