Is sorry - sure upset! (4)
I believe the answer is:
rues
'is sorry' is the definition.
(I've seen this in another clue)
'sure upset' is the wordplay.
'upset' indicates anagramming the letters.
'sure' is an anagram of 'RUES'.
(Other definitions for rues that I've seen before include "Is sorry for - plants" , "Regrets, to be sure" , "Bemoans" .)