I believe the answer is:
'milne marsupial' 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 "Small inhabitant of Hundred Acre Wood" , "Jumping marsupial" , "Young marsupial" , "Marsupial (informal)" , "Kangaroo (Austr. colloq.)" .)