Pertaining to vision (5)
I believe the answer is:
optic
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">pertaining to vision</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for optic that I've seen before include "Like a sort of nerve" , "Device for measuring gin etc" , "Related to sight" , "Bar measure" , "Relating to the eye" .)