Pig seen in commercial on ship (6)
I believe the answer is:
aboard
'on ship' is the definition.
(aboard a ship)
'pig seen in commercial' is the wordplay.
'pig' becomes 'boar' (male pig).
'seen in' indicates putting letters inside.
'commercial' becomes 'ad' (abbreviation for advertisement).
'boar' inserted into 'ad' is 'ABOARD'.
(Other definitions for aboard that I've seen before include "on a plane, perhaps" , "on deck" , "On a ship, train, plane or other vehicle" , "Ready to sail" , "On (eg, a ship)" .)