Sudden fright (5)
'sudden fright' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for start that I've seen before include "Move quickly < when surprised" , "Onset" , "get under way" , "Commencement" , "Jump suddenly" .)
(Other definitions for alarm that I've seen before include "Put the wind up" , "Unnerve" , "Warning bell" , "Sudden apprehension and fear" , "Arouse to a sense of danger" .)