Vehicle in a crack-up? (3)
I believe the answer is:
car
'vehicle' is the definition.
(I know that car is a type of multi-track vehicle)
'in a crack-up?' is the wordplay.
'in' says the answer is hidden in the clue.
'up?' is a reversal indicator (in down clue: letters go upwards).
'rac' is hidden in the letters of 'crack'.
'rac' reversed gives 'CAR'.
(Other definitions for car that I've seen before include "Railway carriage; vehicle" , "4-wheeled vehicle" , "Auto" , "Personal vehicle" , "Conveyance" .)