Smooth tongue (6)
I believe the answer is:
polish
I believe this is a double definition.
'smooth' is the first definition.
('polish' can be a synonym of 'smooth')
'tongue' is the second definition.
(Polish is an example of a language)
(Other definitions for polish that I've seen before include "Beeswax, for example" , "Rub - refinement" , "Shine with cloth,buff" , "Give shine to" , "Substance used to produce shine" .)