Army doctor's source of milk in capital (6)
I believe the answer is:
moscow
'capital' is the definition.
(Moscow is an example)
'army doctor's source of milk' is the wordplay.
'army doctor's' becomes 'mos'.
'source of milk' becomes 'cow' (I've seen this before).
'mos'+'cow'='MOSCOW'
'in' acts as a link.
(Other definitions for moscow that I've seen before include "capital city" , "City of 13" , "Capital of Russia" , "Bolshoi Ballet base" , "Kremlin location" .)