In the morning gets second supply of missiles perhaps (4)
I believe the answer is:
ammo
'supply of missiles perhaps' is the definition.
'in the morning gets second' is the wordplay.
'in the morning' becomes 'am'.
'gets' says to put letters next to each other.
'second' becomes 'mo' (both are short periods of time).
'am'+'mo'='AMMO'
(Other definitions for ammo that I've seen before include "Bullets (sl.)" , "Bullets, shells, rockets etc" , "eg Bullets, shells" , "Projectiles" , "Slugs perhaps" .)