Defy sister, out of order (6)
I believe the answer is:
resist
'defy' is the definition.
('resist' can be a synonym of 'defy')
'sister out of order' is the wordplay.
'out of order' is an anagram indicator.
'sister' with letters rearranged gives 'RESIST'.
(Other definitions for resist that I've seen before include "Withstand, oppose" , "Withstand a force" , "Withstand or defy" , "Check; withstand" , "Use force to prevent something from happening to sister" .)