Transparent covering of the eye (6)
I believe the answer is:
cornea
'transparent covering of the eye' is the definition.
(I've seen this before)
This is the entire clue.
(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" .)