Polish is the spoken language of the Baltic (6)
I believe the answer is:
finish
'polish' is the definition.
(as in polishing or finishing a surface)
'is the spoken language of the baltic' is the wordplay.
I cannot quite understand how this works, but
'is' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for finish that I've seen before include "Patina" , "perfection" , "Bring to an end" , "Bring to an close" , "Come to an end" .)