Joey in a Milne book (3)
I believe the answer is:
roo
'joey in a milne book' 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 "Kanga's child (Milne)" , "Hopper" , "Kanga's baby (Pooh)" , "bounder" , "short jumper" .)