Hundred Acre Wood youngster (3)
I believe the answer is:
roo
'hundred acre wood 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 "Kangaroo (Austr. colloq.)" , "short jumper" , "Kanga's child (Pooh)" , "Marsupial (abbrev.)" , "Kanga's child (Milne)" .)