Retain different eye part (6)
'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 "Membrane that can become detached", "part of the water pump", "Sensitive part of eye", "Light-sensitive eyeball tissue", "something that helps conjure up image".)