Bird out of its tree? (6)
I believe the answer is:
cuckoo
I believe this is a double definition.
'bird' is the first definition.
(cuckoo is a kind of bird)
'out of its tree?' is the second definition.
(to be out of one's tree is to be mad)
(Other definitions for cuckoo that I've seen before include "Bird that fosters out its eggs" , "that can emerge at hourly intervals" , "Bats" , "flier?" , "fool" .)