Ill-mannered guy (3)
I believe the answer is:
cad
'ill-mannered guy' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for cad that I've seen before include "Stinker" , "A rotter, by Jove" , "Blackguard" , "Wretch" , "A knave, real rotter" .)