WordNet sense 1 (fear resulting from the awareness of danger):

consternation, dismay

WordNet sense 2 (an automatic signal (usually a sound) warning of danger):

alert, alarum, warning signal

WordNet sense 3 (a clock that wakes sleeper at preset time):

alarm clock

WordNet sense 4 (a device that signals the occurrence of some undesirable event):

alarm system, warning device

WordNet sense 5 (fill with apprehension or alarm; cause to be unpleasantly surprised):

appal, appall, horrify