Transparent covering of the eye (6)
I believe the answer is:
cornea
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">transparent covering of the eye</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for cornea that I've seen before include "Transparent part of the eye" , "It covers the eye" , "Transparent outer covering of eyeball" , "Front covering of eye" , "Clear part of the eye" .)