Onomatopoeic bird (6)
I believe the answer is:
cuckoo
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">onomatopoeic bird</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)