Complete language, bar one noun (6)
I believe the answer is:
finish
'complete' is the definition.
(I know that complete can be written as finish)
'language bar one noun' is the wordplay.
'language' becomes 'finnish' (language of Finland).
'bar' indicates named letters should be taken away.
'one noun' becomes 'n'.
'finnish' with 'n' taken out is 'FINISH'.
(Other definitions for finish that I've seen before include "Refinement in social graces" , "Patina" , "Polish, perhaps" , "Terminate, conclude" , "Deal with" .)