Perfect language (6)
I believe the answer is:
polish
I believe this is a double definition.
'perfect' is the first definition.
(polishing is a kind of perfecting)
'language' is the second definition.
(Polish is a language)
(Other definitions for polish that I've seen before include "Beeswax, for example" , "European - gloss" , "Substance used to produce shine" , "taught in finishing school?" , "Language (linked to spitting?)" .)