Cocktail attachment (7)
I believe the answer is:
sidecar
I believe this clue is a double definition.
'cocktail' is the first definition.
(I know that sidecar is a type of cocktail)
'attachment' is the second definition.
(vehicle attached to the side of another)
(Other definitions for sidecar that I've seen before include "Tipple" , "Attachment to motorcycle to accommodate passenger" , "Cocktail; bike attachment" , "Passenger carrier attached to a motorcycle" , "Motorcycle accessory -- cocktail" .)