Frighten Capone with gun (5)
I believe the answer is:
alarm
'frighten' is the definition.
(alarming is a kind of frightening)
'capone with gun' is the wordplay.
'capone' becomes 'Al' (gangster Al Capone).
'with' is a charade indicator (letters next to each other).
'gun' becomes 'arm' (I've seen this in other clues).
'al'+'arm'='ALARM'
(Other definitions for alarm that I've seen before include "Frighten or startle" , "Sudden apprehension and fear" , "Frighten, scare" , "Warning sound or signal" , "It signals danger" .)