Fill with apprehension (5)
(Other definitions for alarm that I've seen before include "Distress signal" , "Noise that wakes" , "Warning bell" , "Burglar deterrent" , "Flap" .)
'fill with apprehension' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for dismay that I've seen before include "Sadden" , "Appal" , "upset" , "Horrified amazement" , "Feeling of despair in the face of obstacles" .)