Boy concealing girl's cruelty (6)
I believe the answer is:
sadism
'cruelty' is the definition.
(deriving pleasure from cruelty)
'boy concealing girl's' is the wordplay.
'boy' becomes 'sam' (boy's name).
'concealing' means one lot of letters goes inside another.
'girl's' becomes 'Dis' (short for Diane or Diana).
'sam' placed around 'dis' is 'SADISM'.
(Other definitions for sadism that I've seen before include "Dim ass (anag)" , "The taking of pleasure in the infliction of pain" , "The love of cruelty" , "Taking pleasure in cruelty" , "Love of inflicting pain" .)