Optical device - French city (4)
I believe the answer is:
lens
I believe this clue is a double definition.
'optical device' is the first definition.
(lens is a kind of optical device)
'french city' is the second definition.
(Lens is a French city)
(Other definitions for lens that I've seen before include "Optical element" , "something to see right through" , "Through which to view" , "One focusing light" , "Optical refractor" .)