Church service (8)
'church service' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for evensong that I've seen before include "6 [SERVICE] ?" , "Church service late in the day" , "... Prayers" , "Anglican service" , "Religious service" .)
'church service' is the definition.
(Mass is a type of church service)
This is the entire clue.
(Other definitions for mass that I've seen before include "Religious ceremony" , "Lump of material" , "congregation for > service" , "Large number of people" , "Matter; large amount" .)