Before noon, doctor's rounds (4)
I believe the answer is:
ammo
'rounds' is the definition.
(rounds of ammunition)
'before noon doctor's' is the wordplay.
'before noon' becomes 'AM' (as in AM on a clock).
'doctor' becomes 'mo' (medical officer).
'am'+'mo'='AMMO'
(Other definitions for ammo that I've seen before include "Bullets, etc. (colloq.)" , "Bullets, say, in short" , "that goes in magazine: ...?" , "What's fired from guns (abbr)" , "Bullets and bombs, etc (abbr)" .)