Oaths sure upset a writer! (9)
I believe the answer is:
authoress
'a writer' is the definition.
(I know that authoress is a type of writer)
'oaths sure upset' is the wordplay.
'upset' indicates an anagram.
'oaths'+'sure'='oathssure'
'oathssure' is an anagram of 'AUTHORESS'.
(Other definitions for authoress that I've seen before include "Female writer" , "Agatha Christie, perhaps" , "Christie possibly" .)