Huge one going last into saltwater vessel (10)
I believe the answer is:
brigantine
'vessel' is the definition.
(brigantine is a kind of vessel)
'huge one going last into saltwater' is the wordplay.
'huge one going' becomes 'igan' (I can't justify this - if you can you should give a lot more credence to this answer).
'last' becomes 't' (I've seen this in other clues).
'into' indicates putting letters inside.
'saltwater' becomes 'brine' (synonyms).
'igan'+'t'='igant'
'igant' inserted into 'brine' is 'BRIGANTINE'.
Can you help me to learn more?
(Other definitions for brigantine that I've seen before include "vessel" , "Boat" , "Two-masted sailing ship" .)