Advantage in ship: Lusitania (4)
I believe the answer is:
plus
'advantage' is the definition.
(a plus is an advantage or benefit)
'in ship lusitania' is the wordplay.
'in' indicates a hidden word.
'PLUS' is hidden within 'ship lusitania'.
(Other definitions for plus that I've seen before include "Something more" , "Advantage; also" , "To the French it's more" , "Together with; advantage" , "Coupled with" .)