Optical glass (4)
I believe the answer is:
lens
'optical glass' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for lens that I've seen before include "Bender of beams" , "Focussing glass" , "Part of the inner eye, or camera" , "Dark cloud; shining light" , "Piece of glass, usually curved" .)