Its passengers have already departed (6)
I believe the answer is:
hearse
'its passengers have already departed' is the definition.
(the passengers of a hearse are dead)
This is the entire clue.
(Other definitions for hearse that I've seen before include "Undertaker's vehicle" , "Car for carrying a coffin" , "Vehicle bearing coffin" , "Vehicle for carrying a coffin" , "Funeral car" .)