A large weapon causes fear (5)
I believe the answer is:
alarm
'fear' is the definition.
(alarm is a kind of fear)
'a large weapon' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'weapon' becomes 'arm' (synonyms).
'a'+'l'+'arm'='ALARM'
'causes' acts as a link.
(Other definitions for alarm that I've seen before include "Warning sound/device" , "Noise giving warning" , "Warning of danger" , "Frighten, scare" , "Burglar deterrent" .)