Unauthorised passenger (8)
I believe the answer is:
stowaway
'unauthorised passenger' is the definition.
(eg on a ship)
This is the entire clue.
(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" .)