Withstand drunken sister (6)
I believe the answer is:
resist
'withstand' is the definition.
('resist' can be a synonym of 'withstand')
'drunken sister' is the wordplay.
'drunken' is an anagram indicator.
'sister' anagrammed gives 'RESIST'.
(Other definitions for resist that I've seen before include "Endure" , "Hold out for sister" , "'Withstand, fight against (6)'" , "Check; withstand" , "Stand firm against" .)