Established church driven out of French region (6)
I believe the answer is:
proven
'established' is the definition.
('proven' can be similar in meaning to 'established')
'church driven out of french region' is the wordplay.
'church' becomes 'ce' (abbreviation for Church of England).
'driven out of' indicates named letters should be taken away.
'french region' becomes 'provence' (I've seen this before).
'provence' with 'ce' taken away is 'PROVEN'.
(Other definitions for proven that I've seen before include "Established with evidence" , "Established beyond doubt" , "Adjective meaning demonstrated beyond doubt" , "Conclusively demonstrated by evidence" .)