River cut off any number (6)

I believe the answer is:

severn

'river' is the definition.
(I've seen this before)

'cut off any number' is the wordplay.
'cut off' becomes 'sever' (I've seen this before).
'any number' becomes 'n' (I've seen this in other clues).
'sever'+'n'='SEVERN'

(Other definitions for severn that I've seen before include "UK's longest river" , "Longest British river" , "River veers N" , "British banker" , "River of England and Wales" .)

I've seen this clue in The Telegraph and The Guardian.

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