Regrets causing sure chaos (4)
I believe the answer is:
rues
'regrets' is the definition.
(I've seen this before)
'sure chaos' is the wordplay.
'chaos' is an anagram indicator.
'sure' with letters rearranged gives 'RUES'.
'causing' acts as a link.
(Other definitions for rues that I've seen before include "Bemoans" , "Regrets, to be sure" , "Is sorry for - plants" .)