They're transparent in the eyes (6)
I believe the answer is:
lenses
'they're transparent in the eyes' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for lenses that I've seen before include "They're transparent in the eyes" , "Focussing glasses" , "They could help you see" , "Optical elements" , "The glasses in spectacles" .)