Bullets (4)
'bullets' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ammo that I've seen before include "Bullet" , "round" , "Bullets, shells, etc. (abbr.)" , "Bullets (sl.)" , "Bullets etc.(informal)" .)
(Other definitions for ammunition that I've seen before include "shot" , "possibly in magazines" , "Bullets, eg" , "Bullets and shells" , "Points used to advantage" .)