Refinement of language (6)
I believe the answer is:
polish
I believe this clue is a double definition.
'refinement' is the first definition.
(I know that refinement can be written as polish)
'language' is the second definition.
(Polish is a language)
'of' acts as a link.
(Other definitions for polish that I've seen before include "Europeans" , "A West Slavic language" , "Smooth or shine by rubbing" , "Substance used to produce shine" , "Give shine to" .)