Russian officer's milk supplier (6)
I believe the answer is:
moscow
'russian' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'officer's milk supplier' is the wordplay.
'officer's' becomes 'mos' (I can't justify this - if you can you should give a lot more credence to this answer).
'milk supplier' becomes 'cow' (I've seen this in another clue).
'mos'+'cow'='MOSCOW'
Can you help me to learn more?
(Other definitions for moscow that I've seen before include "Russian city" , "Capital of Russia" , "Kremlin location" , "Bolshoi Ballet base" , "City of 13" .)