Supply conclusive evidence the clan altered French dialect (9)
I believe the answer is:
provencal
'dialect' is the definition.
(I've seen this before)
'supply conclusive evidence the clan altered french' is the wordplay.
I cannot really understand how this works, but
'evidence' could be 'prove' (synonyms) and 'prove' is located in the answer.
an anagram of 'clan' is 'ncal' which is located in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for provencal that I've seen before include "French citizen" , "of Langue d'oc" , "dialect" , "of southern France" , "Language in France" .)