Service in ship by master (4)
I believe the answer is:
mass
'service' is the definition.
(I've seen this in other clues)
'ship by master' is the wordplay.
'ship' becomes 'ss'.
'by' means one lot of letters go next to another.
'master' becomes 'ma'.
'ss' put after 'ma' is 'MASS'.
'in' is the link.
(Other definitions for mass that I've seen before include "Bulk - RC service" , "Bulk; religious service" , "Total" , "13 [BULK]" , "Matter; large amount" .)