During safest lap, surprisingly, vehicle at the back crashed out (4)
'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 "firm to > go without" , "Speedily - secured" , "Accelerated" , "Can't move" , "Period during which fare is reduced" .)
'out' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Another definition for fast asleep that I've seen is " Slumbering soundly".)