A bit of a rotter, a scallywag, a knave (6)
I believe the answer is:
rascal
'knave' is the definition.
(I know that knave is a class)
'a bit of a rotter a scallywag a' is the wordplay.
'a bit of' says the answer is hidden in the clue.
'RASCAL' is hidden within 'rotter a scallywag a'.
(Other definitions for rascal that I've seen before include "Affectionate term for imp" , "not one to be admired" , "Impudent person" , "Rogue of a lascar" , "mischief-maker" .)