Small animal in river with two ducks (3)

I believe the answer is:

roo

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'small animal' is the definition.
'roo' can be an answer for 'animal' (I've seen this before). I'm unsure of the 'small' bit.

'in river with two ducks' is the wordplay.
'two ducks' can mean two substitutions for 'duck'.
'in' indicates putting letters inside.
'river' becomes 'r' (abbreviation for river).
'with' means one lot of letters go next to another.
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'duck' becomes 'o' (resembles zero - 'duck' in cricket).
'r'+'o'='ro'
'ro' placed around 'o' is 'ROO'.

Can you help me to learn more?

(Other definitions for roo that I've seen before include "Small inhabitant of Hundred Acre Wood" , "Young marsupial" , "Kanga's child (Milne)" , "Wild animal" , "Marsupial (Aus. colloq.)" .)

I've seen this clue in The Sun.

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