Mass at church with the French saint is without equal (9)
I believe the answer is:
matchless
'without equal' is the definition.
(thesaurus)
'mass at church with the french saint' is the wordplay.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'church' becomes 'ch' (common abbreviation).
'with' is a charade indicator (letters next to each other).
'the french' becomes 'les' (les is the in french).
'saint' becomes 's'.
'm'+'at'+'ch'+'les'+'s'='MATCHLESS'
'is' acts as a link.
(Other definitions for matchless that I've seen before include "reason for rubbing sticks together?" , "Like the top" , "Unbeatable" , "Perfect" , "Second to none" .)