Dreadful chaos, mostly — very? (5)
'dreadful chaos mostly very?' is the definition.
I can't judge whether this definition defines the answer.
'dreadful chaos mostly very?' is the wordplay.
'dreadful' is an anagram indicator.
'mostly' means to remove the last letter.
'very?' becomes 'v' (abbreviation).
'chaos' with its final letter removed is 'chao'.
'chaov' with letters rearranged gives 'HAVOC'.
(Other definitions for havoc that I've seen before include "Violent and needless disturbance", "Mayhem", "desolation", "Disaster", "Ruin".)