Callous (9)
I believe the answer is:
heartless
'callous' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for heartless that I've seen before include "Lacking pity due to cardiac deficiency" , "Lacking feeling through cardiac deficiency" , "Without pity or feeling, being defective in cardiac way" , "Unkind" , "Scrooge was" .)