Winnie-the-Pooh's associate rhymester (3)

I believe the answer is:

roo

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'rhymester' is the definition.
Both the definition and answer are singular nouns.
Perhaps they are linked in a way I don't understand?

I cannot understand how the remainder of the clue works.

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(Other definitions for roo that I've seen before include "Marsupial (informal)" , "Kanga's child (Pooh)" , "Kangaroo (Austr. colloq.)" , "Diminutive inhabitant of Hundred Acre Wood" , "Marsupial (dim.)" .)

I've seen this clue in the Evening Standard.

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