Medical officer supports morning rounds (4)
I believe the answer is:
ammo
'rounds' is the definition.
(rounds of ammunition)
'medical officer supports morning' is the wordplay.
'medical officer' becomes 'MO' (abbreviation).
'supports' means one lot of letters go next to another (bottom letters provide a support for the top ones).
'morning' becomes 'am' (as in 12:00 AM).
'mo' after 'am' is 'AMMO'.
(Other definitions for ammo that I've seen before include "Bullets, shells (colloq.)" , "eg Bullets, shells" , "that goes in magazine: ...?" , "Bullets, etc. (colloq.)" , "Bullets, say, in short" .)