High quality former monk's room in French church (10)
I believe the answer is:
excellence
'high quality' is the definition.
(excellence is a kind of high quality)
'former monk's room in french church' is the wordplay.
'former' becomes 'ex' (prefix meaning former or onetime).
'monk's room' becomes 'cell' (I've seen this in another clue).
'in french' becomes 'en' ('in' in French).
'church' becomes 'CE' (abbreviation for Church of England).
'ex'+'cell'+'en'+'ce'='EXCELLENCE'
(Other definitions for excellence that I've seen before include "Outstanding quality" , "Ambassadorial" , "greatness" , "Exceptional quality" , "Great merit" .)