Showing no mercy (8)
I believe the answer is:
ruthless
'showing no mercy' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ruthless that I've seen before include "Lacking pity or mercy" , "Having no mercy like hustlers" , "Without pity or compassion" , "Hard-hearted" , "Callous" .)