A river is a beauty (6)

I believe the answer is:

adonis

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

'a river is' is the wordplay.
'river' becomes 'don' (I've seen this before).
'a'+'don'+'is'='ADONIS'

(Other definitions for adonis that I've seen before include "Handsome youth loved by Aphrodite" , "Type of male beauty" , "Aphrodite's beloved" , "Hunk" , "Archetype of male beauty" .)

I've seen this clue in The Telegraph.

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