Vessel's gone out with maximum on board (7)
I believe the answer is:
galleon
'vessel's' is the definition.
(galleon is a kind of vessel)
'gone out with maximum on board' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'with maximum' becomes 'all' (I am not sure about this - if you are sure you should believe this answer much more).
'on board' means one lot of letters goes inside another.
'gone' is an anagram of 'geon'.
'geon' placed around 'all' is 'GALLEON'.
Can you help me to learn more?
(Other definitions for galleon that I've seen before include "Historical warship with three or more masts" , "Trader" , "Large old square-rigged sailing ship" , "Old Spanish ship" , "Spanish treasure ship" .)