Cathedral city where church converted Sartre (8)
I believe the answer is:
chartres
'cathedral city' is the definition.
(French city with a cathedral)
'church converted sartre' is the wordplay.
'church' becomes 'ch' (common abbreviation).
'converted' indicates an anagram.
'sartre' with letters rearranged gives 'artres'.
'ch'+'artres'='CHARTRES'
'where' is the link.
(Other definitions for chartres that I've seen before include "French city with a cathedral renowned for its stained glass windows" , "Beauce town, Gothic cathedral" , "French town noted for its Gothic cathedral" , "French town with famous cathedral" , "N French city with a distinctive Gothic cathedral" .)