Lightweight motorcycles (6)
I believe the answer is:
mopeds
'lightweight motorcycles' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for mopeds that I've seen before include "Light motor cycles" , "Lightweight motorcycles" , "Low-powered motorbike" , "Low-powered motor-cycles" , "Low-powered motorbikes" .)