Automobile in a crack-up! (3)
I believe the answer is:
car
'automobile' is the definition.
(I know that automobile can be written as car)
'in a crack-up' is the wordplay.
'in' indicates the answer is hidden within the clue.
'up' is a reversal indicator (in down clue: letters go upwards).
'rac' is hidden within 'crack'.
'rac' back-to-front is 'CAR'.
(Other definitions for car that I've seen before include "Road vehicle; rail coach" , "One for the road?" , "4-wheeled vehicle" , "Railway carriage; vehicle" , "Private vehicle" .)