Daffy — bird (6)
I believe the answer is:
cuckoo
I believe this clue is a double definition.
'daffy' is the first definition.
(I've seen this before)
'bird?' is the second definition.
(cuckoo is a kind of bird)
(Other definitions for cuckoo that I've seen before include "It lays eggs in another's 15 across" , "Intruder bird in the nest" , "Cracked" , "Crazy (colloq.); bird" , "building no nest?" .)