Prom night vehicle (4)
I believe the answer is:
limo
'prom night vehicle' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for limo that I've seen before include "Set of rooms" , "Luxurious car (colloq.)" , "vehicle" , "Stretched car (abbr)" , "Large luxurious car" .)