Cunning vehicle's backing in, pronto (7)
I believe the answer is:
smartly
'pronto' is the definition.
(thesaurus)
'cunning vehicle's backing in' is the wordplay.
'cunning' becomes 'sly' ('sly' can be a synonym of 'cunning').
'vehicle's backing' becomes 'mart' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'sly' placed around 'mart' is 'SMARTLY'.
Can you help me to learn more?
(Other definitions for smartly that I've seen before include "How the chic are turned out" , "at a brisk pace" , "pronto" , "wasting no time" .)