Distance in which automobile reverses is a marvel (7)
I believe the answer is:
miracle
'a marvel' is the definition.
(thesaurus)
'distance in which automobile reverses' is the wordplay.
'distance' becomes 'mile'.
'in which' means one lot of letters goes inside another.
'automobile' becomes 'car' ('car' can be a synonym of 'automobile').
'reverses' shows that the letters should be reversed in order.
'car' written backwards gives 'rac'.
'mile' placed around 'rac' is 'MIRACLE'.
'is' acts as a link.
(Other definitions for miracle that I've seen before include "something amazing" , "Supernatural happening" , "Extraordinary event" , "Amazing event" , "Divine action" .)