Crossword Genius

Flattens northern hills (5)

Ross

I believe the answer is:

fells

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

'hills' is the definition.
The definition and answer can be both man-made objects as well as being plural nouns.
Maybe you can see a link between them that I don't see?

I can't explain the remainder of the clue.

Can you help me to learn more?

I've seen this clue in The Times.
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