Crossword Genius

Valueless river? (4)

Ross

I believe the answer is:

ouse

'river?' is the definition.
(Yorkshire river)

I cannot understand how the rest of the clue works.

(Other definitions for ouse that I've seen before include "One of several rivers" , "N English river (and S English river)" , "Great river at King's Lynn" , "Runner" , "River flowing through York" .)

I've seen this clue in The Guardian.
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