Naughty drivers close to boundary (4)
I believe the answer is:
racy
'naughty' is the definition.
('racy' can be a synonym of 'naughty')
'drivers close to boundary' is the wordplay.
'drivers' becomes 'rac' (I've seen this in other clues).
'close to' says to take the final letters (the close/ending of).
The final letter of 'boundary' is 'y'.
'rac'+'y'='RACY'
(Other definitions for racy that I've seen before include "bawdy quality" , "Suggestive, risque" , "rather blue" , "Slightly indecent" , "Sexually suggestive" .)