Retina cell (4)
I believe the answer is:
cone
'retina cell' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for cone that I've seen before include "Geometric solid - fruit of pine" , "children can lick" , "Tapering shape" , "Portable marker for lines on road" , "Movable plastic bollard" .)