Hard-hearted (7)
'hard-hearted' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for callous that I've seen before include "Hard and unfeeling" , "Not feeling" , "Emotionally hardened" , "Cruel or unsympathetic" , "Hard-hearted" .)
(Other definitions for cold-blooded that I've seen before include "Lacking mercy" , "Reptilian; without excuse of passion" , "Without emotion" .)