Motorbike rear seat (7)
I believe the answer is:
pillion
'motorbike rear seat' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pillion that I've seen before include "Seat behind a motorbike rider" , "Passenger seat at rear of motor-cycle" , "Back seat" , "Passenger seat at rear of motorbike" .)