Nullifies horrendous ultimate chaos! (6)
I believe the answer is:
undoes
'nullifies' is the definition.
The answer and definition can be both related to change as well as being verbs in their -s form.
Perhaps you can see a link between them that I don't see?
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for undoes that I've seen before include "Opens fastener or causes the ruin of" , "Loosens or ruins" .)