Optical element (4)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
lens
'optical element' 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 "French city" , "Piece of glass, usually curved" , "Transparent optical device" , "Optical element" , "Light-focusing device" .)