Aussie hopper (3)
I believe the answer is:
'aussie hopper' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for roo that I've seen before include "Kanga's child (Pooh)" , "large animal" , "Marsupial (dim.)" , "Kanga's child (Milne)" , "Aussie jumper!" .)