It contains rods and cones (6)
I believe the answer is:
retina
'it contains rods and cones' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for retina that I've seen before include "Viewer's part" , "Light-sensitive eye lining" , "Membrane covering the back of the eyeball" , "What's behind the humour" , "Bad tear in part of the eye" .)