Cars wrecked by a learner driver - the devil! (6)
I believe the answer is:
rascal
'the devil' is the definition.
(thesaurus)
'cars wrecked by a learner driver' is the wordplay.
'wrecked' indicates anagramming the letters.
'by' is a charade indicator (letters next to each other).
'learner driver' becomes 'l' (learner drivers have L-plates on their cars).
'cars' is an anagram of 'rasc'.
'rasc'+'a'+'l'='RASCAL'
(Other definitions for rascal that I've seen before include "Brat" , "Rogue or cheeky child" , "A naughty chap or bold boy" , "Lascar (anag.)" , "Impudent child" .)