Rounds in the morning for Medical Officer (4)
I believe the answer is:
ammo
'rounds' is the definition.
(rounds of ammunition)
'morning for medical officer' is the wordplay.
'morning' becomes 'am' (as in 12:00 AM).
'for' means one lot of letters go next to another.
'medical officer' becomes 'MO' (abbreviation).
'am'+'mo'='AMMO'
'in the' is the link.
(Other definitions for ammo that I've seen before include "that goes in magazine: ...?" , "Explosive devices" , "essential supplies for soldiers" , "Bullets, shells (colloq.)" , "Bullet" .)