Withstand sister in confusion (6)
I believe the answer is:
resist
'withstand' is the definition.
(synonyms)
'sister in confusion' is the wordplay.
'in confusion' indicates anagramming the letters.
'sister' with letters rearranged 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" .)