Language buff (6)
I believe the answer is:
polish
This clue is a double definition.
'language' is the first definition.
(I know that Polish is a type of Slavic)
'buff' is the second definition.
(I know that buff can be written as polish)
(Other definitions for polish that I've seen before include "Refine, language" , "Rub to a gloss" , "Buff - from Poland" , "Give shine to" , "Make smooth and glossy" .)