Most ill-mannered (6)
I believe the answer is:
rudest
'most ill-mannered' is the definition.
('rude' can be a synonym of 'ill-mannered')
This is all the clue.
(Other definitions for rudest that I've seen before include "Most bad mannered!" , "Least polite, most unmannerly" , "Most impolite" , "Most ill-mannered" , "Most unrefined" .)