One animal never out of bounds? (8)

I believe the answer is:

kangaroo

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'one animal' is the definition.
(kangaroo is a kind of animal)

I can't explain the rest of the clue.

(Other definitions for kangaroo that I've seen before include "Large leaping mammal" , "Unofficial court of law" , "Large Australian marsupial" , "Marsupial from down-under" , "Bounding marsupial" .)

I've seen this clue in The Sun.

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