Religious service (4)
'religious service' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for mass that I've seen before include "congregation for > service" , "congregate for > service" , "Church rite; large group" , "Widespread" , "Religious ceremony" .)
(Other definitions for rite that I've seen before include "Ceremonial act" , "(Words used in) ceremonial" , "something like mass" , "Customary observance" , "Uncomplaining person" .)