In a crack-up, maybe a write-off (3)
I believe the answer is:
car
'a write-off' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'in a crack-up' is the wordplay.
'in' says the answer is hidden in the clue.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'rac' is hidden within 'crack'.
'rac' backwards is 'CAR'.
'maybe' acts as a link.
Can you help me to learn more?
(Other definitions for car that I've seen before include "estate, perhaps" , "One for the road?" , "Auto" , "Motor vehicle" , "Wheeled vehicle" .)