Real Dictionary

cuckoo clock
countable noun: 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 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
noun: a wall or shelf clock, often carved and decorated, that announces the hours by a sound like the call of the cuckoo, usually accompanied by the appearance of an imitation bird through a little door

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