Frightened by Capone with gun (7)
I believe the answer is:
alarmed
'frightened' is the definition.
(alarming is a kind of frightening)
'by capone with gun' is the wordplay.
'by' means one lot of letters go next to another.
'capone' becomes 'Al' (gangster Al Capone).
'with gun' becomes 'armed' (I've seen this before).
'al' next to 'armed' is 'ALARMED'.
(Other definitions for alarmed that I've seen before include "Frightened, disturbed" , "Perceiving danger" , "Given a turn" , "Worried" , "Experiencing a sudden sense of danger" .)