From Longman Dictionary of Contemporary Englishalarm clockaˈlarm clock noun [countable] 
TMCa clock that makes a noise at a particular time to wake you up
TMCa clock that makes a noise at a particular time to wake you upExamples from the Corpus
alarm clock• Lutine Bell alarm clock by Wherle Decosphere light.• A neighbour had made sure he was awake; there was no alarm clock at home.• I rolled over, sat up, blinked, and glared at the alarm clock.• The alarm clock overhead read 5.28, time for a quick cup of tea before his new shift began.• Voice-control AB313 travel alarm clock showing world time zones, £21.95.• To give you an idea what that means, 80 decibels is generated by a typical alarm clock. 