Mother’s meeting son’s crowd (4)
I believe the answer is:
mass
'crowd' is the definition.
(I know that mass is a type of crowd)
'mother's meeting son's' is the wordplay.
'mother's' becomes 'ma's' (Ma is another word for mother or mum).
'meeting' means one lot of letters go next to another.
'mas'+'s'='MASS'
(Other definitions for mass that I've seen before include "congregate for > service" , "load" , "Substance; a service" , "Large amount, body" , "Religious rite" .)