Motorcycle attachment (7)
I believe the answer is:
sidecar
'motorcycle attachment' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sidecar that I've seen before include "In which a passenger may be taken" , "One-wheeled compartment attached to a motorcycle" , "Attachment to motorcycle to accommodate passenger" , "that's suitable for a non-driver" , "Low-slung two-wheeler" .)