Mother ship in service (4)
I believe the answer is:
mass
'service' is the definition.
(church service)
'mother ship' is the wordplay.
'mother' becomes 'ma' (ma can mean mother).
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'ma'+'ss'='MASS'
'in' acts as a link.
(Other definitions for mass that I've seen before include "RC ceremony" , "Throng" , "Bulk; religious service" , "Lump of material" , "load" .)