Medical officer on a vessel in Russia (6)
I believe the answer is:
moscow
'in russia' is the definition.
('Moscow' is part of 'russia')
'medical officer on a vessel' is the wordplay.
'medical officer' becomes 'MO' (abbreviation).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'a vessel' becomes 'scow' (scow is a kind of vessel).
'mo'+'scow'='MOSCOW'
(Other definitions for moscow that I've seen before include "Russian city" , "City of 13" , "Kremlin location" , "Olympic city, 1980" , "Bolshoi Ballet base" .)