Old marsupial's back down on unknown river (7)

I believe the answer is:

dasyure

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'old marsupial's' is the definition.
'dasyure' can be an answer for 'marsupial's' (dasyure is a kind of marsupial). I am not sure about the 'old' bit.

'back down on unknown river' is the wordplay.
'back' says the letters should be written backwards.
'down' becomes 'sad' (I've seen this before).
'on unknown' becomes 'y' (algebra. I am not sure about the 'on' bit.).
'river' becomes 'Ure' (I have seen 'River of Wensleydale ' mean 'ure' so perhaps 'river' could also mean 'ure').
'sad' in reverse letter order is 'das'.
'das'+'y'+'ure'='DASYURE'

Can you help me to learn more?

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

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