Scaremonger having a large weapon is finally shot (8)
I believe the answer is:
alarmist
'scaremonger' is the definition.
(I've seen this before)
'a large weapon is finally shot' is the wordplay.
'large' becomes 'l'.
'weapon' becomes 'arm' (I've seen this before).
'finally' says to take the final letters.
The last letter of 'shot' is 't'.
'a'+'l'+'arm'+'is'+'t'='ALARMIST'
'having' is the link.
(Other definitions for alarmist that I've seen before include "prophet of doom" , "spreading fear" , "One spreads unnecessary worry or fear" , "Worrying individual" , "unduly frightening" .)