Smooth language (6)
I believe the answer is:
polish
I believe this clue is a double definition.
'smooth' is the first definition.
('polish' can be a synonym of 'smooth')
'language' is the second definition.
(Polish is an example)
(Other definitions for polish that I've seen before include "Buff - from Poland" , "'Buff, burnish (6)'" , "Europeans" , "tongue" , "elegance" .)