All those in for a service (12)
I believe the answer is:
congregation
'service' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Maybe they are linked in a way I don't understand?
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for congregation that I've seen before include "Group of worshippers" , "Group assembling for worship" .)