Retain different eye part (6)
I believe the answer is:
retina
'eye part' is the definition.
('retina' is part of 'eye')
'retain different' is the wordplay.
'different' is an anagram indicator.
'retain' anagrammed gives 'RETINA'.
(Other definitions for retina that I've seen before include "Bad tear in part of the eye" , "Light-sensitive eye lining" , "Light-receptive part of eye" , "Back surface of eye" , "part of the water pump" .)