Extremely cruel act (8)
I believe the answer is:
atrocity
'extremely cruel act' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for atrocity that I've seen before include "Barbarity, inhumanity" , "Evil act" , "Barbarous, inhuman act" , "Heinous deed" , "monstrous crime" .)