Brass covering northern hills (8)

I believe the answer is:

pennines

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'hills' is the definition.
Both the answer and definition are plural nouns.
Perhaps there's a link between them I don't understand?

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

Can you help me to learn more?

(Other definitions for pennines that I've seen before include "Range of hills in England" , "chain of hills" , "English mountain range" , "Backbone of England" .)

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