River flowing into a sewer? (5)

I believe the answer is:

weser

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'river' is the definition.
(German river)

'flowing into a sewer?' is the wordplay.
'sewer' anagrams to 'WESER'.
However, I'm not sure how this is indicated.

Can you help me to learn more?

(Other definitions for weser that I've seen before include "Er, sew up the German river" , "German runner" .)

I've seen this clue in the Evening Standard.

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