Cold-hearted (6)
I believe the answer is:
unkind
'cold-hearted' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for unkind that I've seen before include "Hardhearted" , "Uncaring, nasty" , "Thoughtless -- inhumane" , "Unsympathetic or cruel" , "Not gentle or considerate" .)