Retailer dealing in Leicester and Derby?

I believe the answer is:

cheesemonger

I'm a little stuck... Click here to teach me more about this clue!

'derby?' is the definition.
Both the answer and definition are singular nouns.
Maybe you can see an association between them that I can't see?

I don't understand how the rest of the clue works.

Can you help me to learn more?

(Another definition for cheesemonger that I've seen is " Wensleydale retailer?".)

I've seen this clue in The Telegraph.

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.