Rub smooth tongue (6)
I believe the answer is:
polish
I believe this clue is a triple definition!
'rub' is the first definition.
(I've seen this before)
'smooth' is the second definition.
(synonyms)
'tongue' is the third definition.
(Polish is an example of a language)
(Other definitions for polish that I've seen before include "European language - refinement" , "Rub to make the surface shine" , "make things shiny" , "Beeswax, for example" , "Of one from Warsaw, say" .)