Salmon fishing river rising in the Scottish Borders and flowing into the North Sea at Berwick in Northumberland (5)

I believe the answer is:

tweed

(Other definitions for tweed that I've seen before include "Thick woollen cloth - northern river" , "Running water" , "Woollen material" , "Woollen cloth - Scotland/England border river" , "Rough material" .)

I've seen this clue in The Guardian.

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