Old monk's room in French church? Goodness! (10)
I believe the answer is:
excellence
'goodness' is the definition.
(thesaurus)
'old monk's room in french church?' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'monk's room' becomes 'cell' (I've seen this before).
'in french' becomes 'en' ('in' in French).
'church?' becomes 'CE' (Church of England).
'ex'+'cell'+'en'+'ce'='EXCELLENCE'
(Other definitions for excellence that I've seen before include "greatness" , "Great merit" , "Exceptional quality" , "Ambassadorial" , "Superiority" .)