Sudden fear (5)
(Other definitions for alarm that I've seen before include "with which many awake to in the morning" , "Mild panic" , "Trepidation" , "Panicky arousal" , "Great concern" .)
'sudden fear' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for panic that I've seen before include "mass hysteria?" , "Sudden overpowering fear" , "Have kittens" , "Frantic fear" , "Overwhelming terror" .)