Retain new part of the eye (6)
I believe the answer is:
retina
'part of the eye' is the definition.
('retina' is part of 'eye')
'retain new' is the wordplay.
'new' indicates an anagram.
'retain' anagrammed gives 'RETINA'.
(Other definitions for retina that I've seen before include "Light-sensitive membrane of the eyeball" , "What's behind the humour" , "Membrane covering the back of the eyeball" , "Tear in (anag) - part of the eye" , "Back surface of eye" .)