On a ship or train (6)
I believe the answer is:
aboard
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">on a ship or train</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for aboard that I've seen before include "on gig, perhaps?" , "On vessel or vehicle" , "on a liner, say" , "On an aircraft" , "on base" .)