A doctor crossing miles for rounds (4)
I believe the answer is:
ammo
'for rounds' is the definition.
(rounds of ammunition)
'a doctor crossing miles' is the wordplay.
'doctor' becomes 'mo' (medical officer).
'crossing' means one lot of letters goes inside another (to cross can mean to go over or around).
'miles' becomes 'm' (abbreviation).
'mo' enclosing 'm' is 'mmo'.
'a'+'mmo'='AMMO'
(Other definitions for ammo that I've seen before include "Bullets (sl.)" , "explosives, perhaps" , "eg Bullets, shells" , "Bullets, shells (colloq.)" , "round" .)