Cuckoo making beastly noise (7)
I believe the answer is:
barking
I believe this clue is a double definition.
'cuckoo' is the first definition.
(both can mean mad)
'making beastly noise' is the second definition.
(making the noise of a dog)
(Other definitions for barking that I've seen before include "Completely mad" , "What Battersea resident may be doing" , "6D 5A [TUBE STATION]" , "Mad - town forming a London borough with Dagenham" , "Dog may be" .)