Brush up on tongue (6)
I believe the answer is:
polish
This clue is a double definition.
'brush up' is the first definition.
(synonyms)
'tongue' is the second definition.
(Polish is an example of a language)
'on' acts as a link.
(Other definitions for polish that I've seen before include "tongue" , "Refine, language" , "A West Slavic language" , "make things shiny" , "Rub until shiny" .)