The slurs about being callous (8)
I believe the answer is:
ruthless
'callous' is the definition.
(thesaurus)
'the slurs about' is the wordplay.
'about' indicates anagramming the letters.
'the'+'slurs'='theslurs'
'theslurs' is an anagram of 'RUTHLESS'.
'being' acts as a link.
(Other definitions for ruthless that I've seen before include "Without mercy or pity" , "Without pity, merciless" , "devoid of compassion" , "nasty" , "Unsparing" .)