Imp crashes cars by a lake (6)
I believe the answer is:
rascal
'imp' is the definition.
('rascal' can be a synonym of 'imp')
'crashes cars by a lake' is the wordplay.
'crashes' indicates an anagram.
'by' says to put letters next to each other.
'lake' becomes 'l' (**).
'cars' anagrammed gives 'rasc'.
'rasc'+'a'+'l'='RASCAL'
(Other definitions for rascal that I've seen before include "Brat" , "Scoundrel or mischievous child" , "Horror" , "Lascar (anag.)" , "Rogue, scamp" .)