Motorbike attachment (7)
I believe the answer is:
sidecar
'motorbike attachment' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for sidecar that I've seen before include "Alcoholic drink" , "Cocktail; bike attachment" , "Motorcycle accessory -- cocktail" , "Cocktail of brandy, orange liqueur and lemon juice" , "In which a passenger may be taken" .)