On the motorway, to reverse a car half a mile is a marvel! (7)
I believe the answer is:
miracle
'a marvel' is the definition.
(thesaurus)
'on the motorway to reverse a car half a mile' is the wordplay.
'on' says to put letters next to each other.
'the motorway' becomes 'mi' (resembles M1, English motorway).
'to reverse' says the letters should be written backwards.
'half a' means to take half the letters.
'car' reversed gives 'rac'.
'mile' with half the letters taken is 'le'.
'mi'+'rac' is 'mirac'.
'mirac'+'le'='MIRACLE'
'is' acts as a link.
(Other definitions for miracle that I've seen before include "astonishing feat" , "Supernatural act of God" , "Divinely-inspired event" , "God-inspired event" , "something amazing" .)