Sudden terror (5)
(Other definitions for alarm that I've seen before include "Warning bell" , "It signals danger" , "Trepidation" , "Warning siren" , "Panicky arousal" .)
'sudden terror' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for panic that I've seen before include "Losing one's head" , "Great consternation" , "Contagious fear" , "Frenzy" , "Grass" .)