Barbaric, merciless (5)
I believe the answer is:
cruel
This clue is a double definition.
'barbaric' is the first definition.
(I've seen this in another clue)
'merciless' is the second definition.
(I've seen this in another clue)
(Other definitions for cruel that I've seen before include "Deliberately and pitilessly causing pain" , "Merciless" , "Heartless" , "Vicious" , "Painful" .)