Zealot contributing to upturn in pureness evidently (6)
I believe the answer is:
essene
'zealot' is the definition.
(I've seen this before)
'upturn in pureness evidently' is the wordplay.
'upturn' shows that the letters should be reversed in order (in a down clue, letters go up).
'in' indicates the answer is hidden within the clue.
'enesse' is hidden in the letters of 'pureness evidently'.
'enesse' reversed gives 'ESSENE'.
'contributing to' is the link.
(Other definitions for essene that I've seen before include "Zealot" , "Ascetic type" , "Early Jewish ascetic" , "Member of Dead Sea Scrolls sect" , "Old Jewish sectarian" .)