One who enjoys inflicting pain (6)
I believe the answer is:
sadist
'one who enjoys inflicting pain' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sadist that I've seen before include "An amalgam, made up of two or more parts" , "Person who takes pleasure from inflicting pain on others" , "One who enjoys being cruel" , "One gets pleasure from inflicting pain on others" , "'Tis sad s/he likes to inflict pain" .)