Cloth in river at Berwick (5)
I believe the answer is:
tweed
'cloth' is the definition.
(tweed is a kind of cloth)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for tweed that I've seen before include "flows across a border" , "Boundary river" , "Sort of jacket" , "passes between English and Scottish banks" , "Woollen material" .)