Pig placed in trailer on ship? (6)
I believe the answer is:
aboard
'on ship?' is the definition.
(aboard a ship)
'pig placed in trailer' is the wordplay.
'pig' becomes 'boar' (I've seen this before).
'placed in' is an insertion indicator (I've seen this in other clues).
'trailer' becomes 'ad' (short for advert).
'boar' put inside 'ad' is 'ABOARD'.
(Other definitions for aboard that I've seen before include "at sea, say?" , "On (eg, a ship)" , "ready to take off" , "Ready to sail" , "On an aircraft" .)