American swine departs on a ship (6)
I believe the answer is:
aboard
'on a ship' is the definition.
(I've seen this before)
'american swine departs' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'swine' becomes 'boar' (male pig).
'departs' becomes 'd' (common abbreviation e.g. train timetables).
'a'+'boar'+'d'='ABOARD'
(Other definitions for aboard that I've seen before include "Ready to sail" , "'On a ship or train, say (6)'" , "on deck" , "On (a ship/plane)" , "on base" .)