Russian once partly also Vietnamese (6)
I believe the answer is:
soviet
'russian' is the definition.
(I have seen 'Old Russian' mean 'soviet' so perhaps 'russian' could also mean 'soviet')
'partly also vietnamese' is the wordplay.
'partly' indicates the answer is hidden within the clue (part of the clue is used).
'SOVIET' can be found hidden inside 'also vietnamese'.
'once' is the link.
(Other definitions for soviet that I've seen before include "Like the USSR" , "Old government council" , "Old Russian council" , "Former Russian council" , "Russian workers' council, originally" .)