Unauthorised passenger (8)
I believe the answer is:
stowaway
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">unauthorised passenger</span>' is the definition.<br/ ><span class="explanation-format__explanation">(eg on a ship)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for stowaway that I've seen before include "One hides aboard ship to get free passage" , "Hidden, ticketless voyager" , "Travel secretly" , "accommodation, perhaps, for this non-paying traveller?" , "Hider on board" .)