A fool and awfully rude; that's for certain (7)
I believe the answer is:
assured
'for certain' is the definition.
(I've seen this in another clue)
'a fool and awfully rude' is the wordplay.
'a fool' becomes 'ass' (both can mean a stupid person).
'and' says to put letters next to each other.
'awfully' is an anagram indicator.
'rude' anagrammed gives 'ured'.
'ass'+'ured'='ASSURED'
'that's' is the link.
(Other definitions for assured that I've seen before include "Confident and self-possessed" , "Confident - certain" , "Secured" , "without doubt" , "Promised, guaranteed" .)