One may look through this French town (4)
I believe the answer is:
lens
This clue is a double definition.
'one may look through this' is the first definition.
(eg a glasses lens)
'french town' is the second definition.
(town in northern France)
(Other definitions for lens that I've seen before include "Optical refractor" , "One could project a picture" , "Photographer's accessory" , "Refracting device" , "Focussing glass" .)