Frightened American was first to grab weapon (7)
I believe the answer is:
alarmed
'frightened' is the definition.
(alarming is a kind of frightening)
'american was first to grab weapon' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'was first' becomes 'led' (to lead is to go first).
'to grab' means one lot of letters goes inside another.
'weapon' becomes 'arm' (synonyms).
'led' enclosing 'arm' is 'larmed'.
'a'+'larmed'='ALARMED'
(Other definitions for alarmed that I've seen before include "Flapping" , "as a door handle may be?" , "Given a turn" , "Frightened, startled" , "Worried" .)