One practises self-denial as a spiritual discipline (7)
I believe the answer is:
ascetic
'one practises self-denial as a spiritual discipline' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ascetic that I've seen before include "Self-denier" , "Self-denying person who shuns ordinary pleasures" , "One who practises extreme self-discipline" , "One practises self-denial as a spiritual discipline" , "Person who lives severely simple life" .)