Crazy fliers (4)
I believe the answer is:
bats
I believe this clue is a double definition.
'crazy' is the first definition.
(bats can informally mean crazy)
'fliers' is the second definition.
(things that fly)
(Other definitions for bats that I've seen before include "Bonkers (sl.)" , "Night-flying creatures" , "Lunatic" , "Mad - nocturnal creatures" , "Flying nocturnal mammals" .)