During safest lap, surprisingly, vehicle at the back crashed out (4)
I believe the answer is:
Here is my best explanation:
'out' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
I can't explain the remainder of the clue.
(Other definitions for fast that I've seen before include "Speedy", "Of clock, ahead of correct time", "sort of convenience food", "Quick - don't eat", "Rapidly; tight", "Starve; quick", "Moored - fleet", "Quick - stop eating", "Unable to move" and "firm to > go without".)