Polish nationalist abandons language (6)
I believe the answer is:
finish
'polish' is the definition.
(as in polishing or finishing a surface)
'nationalist abandons language' is the wordplay.
'nationalist' becomes 'N' (abbreviation in politics).
'abandons' is a deletion indicator.
'language' becomes 'finnish' (language of Finland).
'finnish' with 'n' taken away is 'FINISH'.
(Other definitions for finish that I've seen before include "Bring to an close" , "Conclude, end" , "Complete or terminate" , "Treatment of surface" , "Deal with" .)