Cold-hearted (6)
'cold-hearted' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for unkind that I've seen before include "Hardhearted" , "Uncaring, nasty" , "Thoughtless -- inhumane" , "Unsympathetic or cruel" , "Not gentle or considerate" .)
'cold-hearted' is the definition.
(thesaurus)
This is the entire clue.
(Other definitions for callous that I've seen before include "Hard-hearted" , "Emotionally hardened" , "Cruel or unsympathetic" , "Not feeling" , "Insensitive, cruel" .)