Regretting mostly creating havoc (4)
I believe the answer is:
ruin
'havoc' is the definition.
(I've seen this in another clue)
'regretting mostly' is the wordplay.
'regretting' becomes 'ruing' ('rue' can be a synonym of 'regret').
'mostly' means to remove the last letter (most but not all of the word).
'ruing' with its final letter taken away is 'RUIN'.
'creating' acts as a link.
(Other definitions for ruin that I've seen before include "fall" , "Utter destruction or downfall" , "dishonour" , "loss of position" , "Spoil, impoverish" .)