So French to be receiving kiss, Monsieur, see (9)
I believe the answer is:
extremely
'so' is the definition.
(thesaurus)
'french to be receiving kiss monsieur see' is the wordplay.
'french to be' becomes 'etre' (I've seen this in another clue).
'receiving' indicates putting letters inside.
'kiss' becomes 'x' (X can represent a kiss).
'monsieur' becomes 'm' (French abbreviation - like Mr for Mister).
'see' becomes 'ely' (see is a place with a cathedral - e.g. Ely).
'etre' placed around 'x' is 'extre'.
'extre'+'m'+'ely'='EXTREMELY'
(Other definitions for extremely that I've seen before include "remarkably" , "To a high degree or extent" .)