Master of vessel expected (6)
I believe the answer is:
subdue
'master' is the definition.
('subdue' can be a synonym of 'master')
'vessel expected' is the wordplay.
'vessel' becomes 'sub' (sub is a kind of vessel).
'expected' becomes 'due' (I've seen this before).
'sub'+'due'='SUBDUE'
'of' is the link.
(Other definitions for subdue that I've seen before include "Moderate" , "Quell" , "Bring under control, tame" , "Overcome, suppress" , "Quash" .)