It's raised at length (5)

I believe the answer is:

ridge

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'length' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for ridge that I've seen before include "high-pressure area" , "Hog's back" , "Peak" , "Hog's-back" , "Line of higher ground" .)

I've seen this clue in the Evening Standard.

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