Photoreceptor cell (4)
I believe the answer is:
cone
'photoreceptor cell' is the definition.
(I know that cone is a type of photoreceptor cell)
This is the entire clue.
(Other definitions for cone that I've seen before include "Hollow object associated with ice cream and motorways" , "Something dropped from tree" , "Movable plastic bollard" , "Tapering shape" , "Solid figure" .)