Vehicle in a crack-up (3)
I believe the answer is:
car
'vehicle' is the definition.
(I've seen this before)
'in a crack-up' is the wordplay.
'in' says the answer is hidden in the clue.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'rac' is hidden in the letters of 'crack'.
'rac' written backwards gives 'CAR'.
(Other definitions for car that I've seen before include "4-wheeled vehicle" , "Conveyance" , "Means of transport" , "estate, perhaps" , "Road vehicle; rail coach" .)