Retain different eye part (6)
I believe the answer is:
retina
'eye part' is the definition.
(I know that retina is part of eye)
'retain different' is the wordplay.
'different' indicates an anagram (letters placed in a different order).
'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" .)