Blackhearted (4)
I believe the answer is:
evil
'blackhearted' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for evil that I've seen before include "Malignant" , "Extremely wicked" , "Great wickedness" , "Unholy" , "Vile (anag) -- really bad" .)