Withstand sister in confusion (6)
I believe the answer is:
resist
'withstand' is the definition.
('resist' can be a synonym of 'withstand')
'sister in confusion' is the wordplay.
'in confusion' is an anagram indicator.
'sister' anagrammed gives 'RESIST'.
(Other definitions for resist that I've seen before include "Sister (anag)" , "Stand up to, fight against" , "Take stand against, oppose" , "Check; withstand" , "Bar" .)