River flows back on another (4)

I believe the answer is:

saar

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'river' is the definition.
Both the answer and definition are singular nouns.
Maybe there's a link between them I don't understand?

'flows back on another' is the wordplay.
'back' suggests the final letters (the letter at the back).
'on' is a charade indicator (letters next to each other).
'another' becomes 'aar' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
The last letter of 'flows' is 's'.
's'+'aar'='SAAR'

Can you help me to learn more?

(Other definitions for saar that I've seen before include "German wine valley" , "River of France and Germany" .)

I've seen this clue in the Sydney Morning Herald.

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