Insensitivity (15)
'insensitivity' is the definition.
(I know that thoughtlessness is a type of unkindness)
This is the entire clue.
(Other definitions for thoughtlessness that I've seen before include "The sort of insensitivity" , "Inconsiderate behaviour" , "that Chairman Mao was not noted for?" .)
'insensitivity' is the definition.
(thesaurus)
This is all the clue.
(Another definition for numbness that I've seen is " Lack of feeling".)