Optical aid - urban centre of the Pas-de-Calais coal basin (4)

I believe the answer is:

lens

(Other definitions for lens that I've seen before include "Camera component" , "One could project a picture" , "Optical element" , "Aid for viewing" , "City of northern France: periscope part" .)

I've seen this clue in The Guardian.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.