Cruelty (6)
'cruelty' is the definition.
(deriving pleasure from cruelty)
This is the entire clue.
(Other definitions for sadism that I've seen before include "Deliberate cruelty" , "Perverse pleasure" , "Enjoyment taken in inflicting pain or cruelty" , "Dim ass (anag)" , "Love of inflicting pain" .)
(Other definitions for meanness that I've seen before include "spite" , "Miserly quality" , "Scrooge-like quality" , "Lack of generosity" , "'Stinginess, parsimony (8)'" .)