Crowd in service seeing in Christmas? (4)
I believe the answer is:
mass
I believe this clue is a double definition.
'crowd' is the first definition.
(I know that mass is a type of crowd)
'service seeing in christmas?' is the second definition.
(as in midnight mass on Christmas Eve)
'in' acts as a link.
(Other definitions for mass that I've seen before include "Celebration of the Eucharist, especially in the RC Church" , "Large number of people" , "Widespread" , "Church service - collection" , "load" .)