Rude child, to lie deviously (8)
I believe the answer is:
impolite
'rude' is the definition.
('impolite' can be similar in meaning to 'rude')
'child to lie deviously' is the wordplay.
'child' becomes 'imp' (I have seen 'Mischievous child' mean 'imp' so perhaps 'child' could also mean 'imp').
'deviously' indicates anagramming the letters (I've seen this in other clues).
'to'+'lie'='tolie'
'tolie' is an anagram of 'olite'.
'imp'+'olite'='IMPOLITE'
(Other definitions for impolite that I've seen before include "Bad-mannered" , "Not courteous or mannerly" , "Rude and uncivil" , "short" , "Unrefined" .)