Blue vehicle reversing close to motorway (4)
I believe the answer is:
racy
'blue' is the definition.
(blue can mean racy or obscene)
'vehicle reversing close to motorway' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'reversing' says the letters should be written backwards.
'close to' says to take the final letters (the close of something is the end).
The final letter of 'motorway' is 'y'.
'car' written backwards gives 'rac'.
'rac'+'y'='RACY'
(Other definitions for racy that I've seen before include "Suggestive, risque" , "Spicy" , "bawdy quality" , "Suggestive; spirited" , "rather blue" .)