Deliberately and pitilessly causing pain (5)
I believe the answer is:
cruel
'deliberately and pitilessly causing pain' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cruel that I've seen before include "Diabolical" , "Barbaric" , "Lucre (anag.)" , "Brutal" , "Devoid of pity" .)