The language for a buff? (6)
I believe the answer is:
polish
I believe this clue is a double definition.
'the language' is the first definition.
(Polish is a language)
'for a buff?' is the second definition.
(I've seen this before)
(Other definitions for polish that I've seen before include "Substance used to produce shine" , "make things shiny" , "Language (linked to spitting?)" , "Rub to a gloss" , "Rub to make the surface shine" .)