Motorcycle attachment (7)
I believe the answer is:
sidecar
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">motorcycle attachment</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(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" .)