Clue: Very, very French — almost all attending Mass in old cathedral city (9)
Answer: EXTREMELY
Explanation: Definition: Very
 ["tres"(French for very/exceedingly) **minus ****its last letter**(almost all) **plus**(attending) M(symbol for "mass" in physics) ] **contained in**(in) [ EX-(prefix signifying "old"/former) + ELY(a cathedral city in England) [.