Crazy bird (6)
This clue is a double definition.
'crazy' is the first definition.
(I've seen this before)
'bird' is the second definition.
(I know that cuckoo is a type of bird)
(Other definitions for cuckoo that I've seen before include "Bird with a two-note call" , "Bananas" , "It takes over other birds' nests" , "It lays eggs in another's 15 across" , "Slightly crazy -- bird" .)
'crazy bird' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for loon that I've seen before include "Fool" , "Simple-minded person" , "US bird is also a silly person?" , "N American diving bird" , "Diver; silly person" .)