In SW England, it's known for its mouth (3)

I believe the answer is:

fal

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'in' is the definition.
Both the answer and definition are adjectives.
Perhaps there's an association between them I don't understand?

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for fal that I've seen before include "Cornish river" , "Flower" .)

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