Withstand sister's plan (6)
I believe the answer is:
resist
'withstand' is the definition.
(synonyms)
'sister's plan' is the wordplay.
'plan' is an anagram indicator (plan can mean organise).
'sister' is an anagram of 'RESIST'.
(Other definitions for resist that I've seen before include "'Withstand, fight against (6)'" , "Withstand, struggle against" , "Refuse to comply" , "Check; withstand" , "Withstand or defy" .)