Motorcycle attachment - or cocktail (7)
I believe the answer is:
sidecar
This clue is a double definition.
'motorcycle attachment' is the first definition.
(I've seen this before)
'cocktail' is the second definition.
(I know that Sidecar is a type of cocktail)
(Other definitions for sidecar that I've seen before include "Where a passenger may go" , "Passenger carrier attached to a motorcycle" , "Motorcycle addition" , "Attachment to motor cycle" , "Tipple" .)