Hustlers can be callous (8)
I believe the answer is:
ruthless
'callous' is the definition.
(I've seen this before)
'hustlers can be' is the wordplay.
'can be' indicates an anagram.
'hustlers' is an anagram of 'RUTHLESS'.
(Other definitions for ruthless that I've seen before include "Lacking pity or mercy" , "Hard-hearted" , "Brutal" , "Hustlers can be without mercy" , "Having no mercy like hustlers" .)