Crossword Genius

During safest lap, surprisingly, vehicle at the back crashed out (4,6)

Ross

I think it could be either:

Which length description are you looking for?:

4

4,6

fast asleep

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'out' is the definition.
(thesaurus)

I can't explain the remainder of the clue.

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(Another definition for fast asleep that I've seen is " Slumbering soundly".)

I've seen this clue in The Guardian.
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