Noble philosopher denying Trinity at the end (6)
I believe the answer is:
aristo
'noble' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
(Other definitions for aristo that I've seen before include "Member of the nobility (abbr)" , "privileged individual" , "one of the best" , "Noble (abbrev.)" , "Owned by the state" .)