Bird – crackers (6)
I believe the answer is:
cuckoo
I believe this clue is a double definition.
'bird' is the first definition.
(I know that cuckoo is a type of bird)
'crackers' is the second definition.
(thesaurus)
(Other definitions for cuckoo that I've seen before include "It lays eggs in the nests of other birds" , "Crackpot bird?" , "slightly mad type" , "Slightly mad - bird" , "Bird which lays eggs in anothers nest" .)