Notice about pig on the train (6)
I believe the answer is:
aboard
'on the train' is the definition.
'notice about pig' is the wordplay.
'notice' becomes 'ad'.
'about' indicates putting letters inside.
'pig' becomes 'boar' (I've seen this before).
'ad' placed around 'boar' is 'ABOARD'.
(Other definitions for aboard that I've seen before include "in a car" , "Ready to sail" , "On an aircraft" , "on a plane, perhaps" , "on a ferry?" .)