Self-confident idiot's dreadfully rude (7)
I believe the answer is:
assured
'self-confident' is the definition.
(I've seen this in another clue)
'idiot's dreadfully rude' is the wordplay.
'idiot' becomes 'ass' (both can mean a stupid person).
'dreadfully' is an anagram indicator (a dreadful spelling).
'rude' anagrammed gives 'ured'.
'ass'+'ured'='ASSURED'
(Other definitions for assured that I've seen before include "Secured" , "Sad ruse (anag.)" , "Confident - certain" , "Confident and self-possessed" , "Beyond doubt" .)