Onomatopoeic bird (6)
I believe the answer is:
cuckoo
'onomatopoeic bird' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for cuckoo that I've seen before include "Slightly crazy -- bird" , "Crackpot bird?" , "counting as bird? Yes and no" , "Bats" , "No home bird, this" .)