Doctor following on from morning rounds (4)
I believe the answer is:
ammo
'rounds' is the definition.
(rounds of ammunition)
'doctor following on from morning' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'following on from' means one lot of letters go next to another (some letters follow others).
'morning' becomes 'am' (as in 12:00 AM).
'mo' put after 'am' is 'AMMO'.
(Other definitions for ammo that I've seen before include "essential supplies for soldiers" , "Bullets, shells, rockets etc" , "rounds briefly" , "Bullets, etc. (colloq.)" , "What's fired from a gun (abbr)" .)