Cocktail compartment for passenger (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)
'compartment for passenger' is the second definition.
(eg a motorcycle sidecar)
(Other definitions for sidecar that I've seen before include "Motorcycle add-on" , "Cocktail of brandy, orange liqueur and lemon juice" , "In which Gromit goes" , "Is raced (anag.)" , "Motorbike passenger attachment" .)