Frightened by gangster with gun (7)
I believe the answer is:
alarmed
'frightened' is the definition.
(alarming is a kind of frightening)
'by gangster with gun' is the wordplay.
'by' means one lot of letters go next to another.
'gangster' becomes 'al' (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 "Terrified" , "Flapping" , "Frightened, scared" , "Perceiving danger" , "Frightened, frightened by danger" .)