Widespread destruction (7)
'widespread destruction' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for havoc that I've seen before include "Carnage" , "Chaotic devastation" , "Chaos, confusion" , "Ruin" , "Widespread destruction and great disorder" .)
(Other definitions for carnage that I've seen before include "Blood bath" , "Bloody slaughter and butchery" , "Mayhem" , "Savage killing of people" , "'Massacre, slaughter (7)'" .)