Fear American with large weapon (5)
I believe the answer is:
alarm
'fear' is the definition.
(alarm is a kind of fear)
'american with large weapon' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'with' is a charade indicator (letters next to each other).
'large' becomes 'L' (eg in clothes sizes).
'weapon' becomes 'arm' (synonyms).
'a'+'l'+'arm'='ALARM'
(Other definitions for alarm that I've seen before include "Noise giving warning" , "Flap" , "Worried perhaps if one rings" , "Warning sound/device" , "Announcement of danger" .)