Assemble the mother ship (4)
I believe the answer is:
mass
'assemble' is the definition.
(I know that mass is a more specific form of the action crowd)
'mother ship' is the wordplay.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'ma'+'ss'='MASS'
'the' acts as a link.
(Other definitions for mass that I've seen before include "Rite; quantity of matter" , "Christian ritual" , "Total" , "Accumulation" , "Church ceremony" .)