Batty (6)
I believe the answer is:
cuckoo
'batty' is the definition.
(I know that batty can be written as cuckoo)
This is the entire clue.
(Other definitions for cuckoo that I've seen before include "Intruder bird in the nest" , "It takes over other birds' nests" , "Crazy (colloq.); bird" , "Onomatopoeic bird" , "Bats" .)