Mother ship's bulk (4)
I believe the answer is:
mass
'bulk' is the definition.
(I know that bulk can be written as mass)
'mother ship's' 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'
(Other definitions for mass that I've seen before include "Heap; religious service" , "Bulk; religious service" , "Body of matter" , "Celebration of the Eucharist, especially in the RC Church" , "Catholic church service" .)