Ship is Titanic , one sinking in the sea (10)
I believe the answer is:
brigantine
'ship' is the definition.
(I know that brigantine is a type of sailing ship)
'one sinking in the sea' is the wordplay.
'one' becomes 'i' (Roman numeral).
'sinking' becomes 'gant' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'the sea' becomes 'brine' (I've seen this in other clues).
'i'+'gant'='igant'
'igant' put into 'brine' is 'BRIGANTINE'.
'is titanic' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
(Other definitions for brigantine that I've seen before include "Boat" , "vessel" , "Two-masted sailing ship" .)