Eye receptor cell (4)
I believe the answer is:
cone
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">eye receptor cell</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for cone that I've seen before include "16 Cornet" , "often full of ice" , "Road furniture!" , "Movable plastic bollard" , "Tapering shape" .)