Modus operandi after morning rounds (4)
I believe the answer is:
ammo
'rounds' is the definition.
(rounds of ammunition)
'modus operandi after morning' is the wordplay.
'modus operandi' becomes 'MO' (common abbreviation).
'after' is a charade indicator (letters next to each other).
'morning' becomes 'am' (as in 12:00 AM).
'mo' put after 'am' is 'AMMO'.
(Other definitions for ammo that I've seen before include "Bullets and bombs, etc (abbr)" , "that goes in magazine: ...?" , "Shot" , "Bullets, shells, rockets etc" , "Slugs perhaps" .)