Buff; language (6)
I believe the answer is:
polish
This clue is a double definition.
'buff' is the first definition.
(I know that buff is a more specific form of buff)
'language' is the second definition.
(Polish is a language)
(Other definitions for polish that I've seen before include "Substance used to produce shine" , "Shine with soft cloth, say silverware" , "Give shine to" , "Europeans" , "taught in finishing school?" .)