Doctor on morning rounds (4)
I believe the answer is:
ammo
'rounds' is the definition.
(I've seen this before)
'doctor on morning' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'on' says to put letters next to each other.
'morning' becomes 'am'.
'mo' after 'am' is 'AMMO'.
(Other definitions for ammo that I've seen before include "Bullets, etc. (colloq.)" , "Shot" , "Bullets, say, in short" , "Bullets (sl.)" , "Projectiles" .)