Lost footing in upland moor (4)

I believe the answer is:

fell

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'moor' is the definition.
(I know that moor can be written as fell)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for fell that I've seen before include "Dropped to the ground" , "Bring down - hill" , "Hew; tumbled" , "Brutal -- high moorland" , "Chop down" .)

I've seen this clue in The Times.

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