A.A. Milne youngster (3)
I believe the answer is:
roo
'a a milne youngster' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for roo that I've seen before include "Marsupial (abbrev.)" , "young skipper" , "short jumper" , "Marsupial (dim.)" , "Wild animal" .)