Eye piece? (4)
'eye piece?' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for iris that I've seen before include "Flag - part of eye" , "Rainbow-goddess" , "Lily" , "Pupil's place?" , "bit of mince pie" .)
(Other definitions for retina that I've seen before include "Part of the eyeball" , "It helps one to see" , "Membrane in eye, sensitive to light" , "Membrane that can become detached" , "Sensitive layer in eye" .)