Withstand naughty sister (6)
I believe the answer is:
resist
'withstand' is the definition.
('resist' can be a synonym of 'withstand')
'naughty sister' is the wordplay.
'naughty' indicates anagramming the letters.
'sister' anagrammed gives 'RESIST'.
(Other definitions for resist that I've seen before include "Use force to prevent something from happening to sister" , "Endure" , "Put up fight, oppose forcefully" , "Combat" , "Oppose or fight against someone" .)