A cuckoo clock is a clock with a door from which a toy cuckoo comes out and makes noises like a cuckoo every hour or half hour.
cuckoo clock in British English
noun
a clock in which a mechanical cuckoo pops out with a sound like a cuckoo's call when the clock strikes
cuckoo clock in American English
a clock with a small toy figure of a cuckoo in it, which pops out at regular intervals, usually on the hour, to the accompaniment of a sound imitating the bird's call