Cannonballs or bullets, for short (4)
I believe the answer is:
ammo
I believe this clue is a double definition.
'cannonballs' is the first definition.
(I've seen this in another clue)
'bullets for short' is the second definition.
(I've seen this in another clue)
(Other definitions for ammo that I've seen before include "Bullets (sl.)" , "arms" , "explosives, perhaps" , "rounds briefly" , "Explosive devices" .)