The glasses in eyeglasses (6)
I believe the answer is:
lenses
'the glasses in eyeglasses' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lenses that I've seen before include "Camera attachments" , "They're transparent in the eyes" , "They're transparent in the eye" , "The glasses in spectacles" , "Optical elements" .)