The ship had cast off (4)
I believe the answer is:
shed
'cast off' is the definition.
(shedding is a kind of casting off)
'the ship had' is the wordplay.
'the ship' becomes 'she' ('she' is often used to refer to a ship).
'had' becomes 'd' (in contractions e.g. 'I'd', 'you'd' etc).
'she'+'d'='SHED'
(Other definitions for shed that I've seen before include "Shack" , "Get rid of - outbuilding" , "In which tools may be" , "Get rid of -- moult" , "Abandon" .)