Bullets a millimetre round (4)
I believe the answer is:
ammo
'bullets' is the definition.
(I've seen this before)
'a millimetre round' is the wordplay.
'millimetre' becomes 'mm' ('mm' can be a synonym of 'millimetre').
'round' becomes 'o' (round can mean a round object).
'a'+'mm'+'o'='AMMO'
(Other definitions for ammo that I've seen before include "arms" , "Bullets (sl.)" , "Bullets, shells, rockets etc" , "rounds briefly" , "Bullets, shells, etc. (abbr.)" .)