Defective auto (5)
I believe the answer is:
lemon
'defective auto' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for lemon that I've seen before include "certainly not a plum!" , "Sharp-tasting fruit" , "Pulsate" , "Fruit - 11 (anag)" , "Unhelpful answer" .)