Frightens almost everyone with weapons (6)
I believe the answer is:
alarms
'frightens' is the definition.
(alarming is a kind of frightening)
'almost everyone with weapons' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'everyone' becomes 'all' (I've seen this before).
'with' means one lot of letters go next to another.
'weapons' becomes 'arms' ('arm' can be a synonym of 'weapon').
'all' with its last letter removed is 'al'.
'al'+'arms'='ALARMS'
(Other definitions for alarms that I've seen before include "Danger signals" , "Flaps" , "Frightens like warning devices" , "Frightens or alerts" , "Fills with apprehension" .)