Come together in lump (4)
I believe the answer is:
mass
This clue is a double definition.
'come together' is the first definition.
(I've seen this in another clue)
'lump' is the second definition.
(I have seen 'Lump of material' mean 'mass' so perhaps 'lump' could also mean 'mass')
'in' is the link.
(Other definitions for mass that I've seen before include "Church service - collection" , "Lump of material" , "Lots of people" , "congregate for > service" , "Ceremony for many" .)