Inevitable in creating endless havoc! (7)
I believe the answer is:
certain
'inevitable' is the definition.
('certain' can be similar in meaning to 'inevitable')
'in creating endless havoc' is the wordplay.
I cannot quite see how this works, but
'in' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for certain that I've seen before include "Assured" , "Known beyond doubt" , "Nice art (anag)" , "Sure, definite" , "Sure; specific" .)