Long distance in which vehicle reversed, a marvel (7)
I believe the answer is:
miracle
'a marvel' is the definition.
(I've seen this in another clue)
'long distance in which vehicle reversed' is the wordplay.
'long distance' becomes 'mile'.
'in which' indicates putting letters inside.
'vehicle' becomes 'car' (car is a kind of vehicle).
'reversed' shows that the letters should be reversed in order.
'car' backwards is 'rac'.
'mile' enclosing 'rac' is 'MIRACLE'.
(Other definitions for miracle that I've seen before include "Divine action" , "Amazing event" , "Unexplained wonder" , "Extraordinary event" , "astonishing feat" .)