The ship would cast off (4)
I believe the answer is:
shed
'cast off' is the definition.
(synonyms)
'the ship would' is the wordplay.
'the ship' becomes 'she' ('she' is often used to refer to a ship).
'would' becomes 'd' (I've seen this in other clues).
'she'+'d'='SHED'
(Other definitions for shed that I've seen before include "Garden feature" , "Take off in hut?" , "Get rid of - hut" , "dropped" , "Abandon" .)