River flow that's sluggish, you'd say (4)

I believe the answer is:

ouse

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'river' is the definition.
(Yorkshire river)

I don't understand how the rest of the clue works.

(Other definitions for ouse that I've seen before include "River at Lewes (Sussex)" , "Norfolk river" , "bird has lost its tail" , "Runner" , "Flowing water" .)

I've seen this clue in The Independent.

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