Fight sister’s plan (6)
I believe the answer is:
resist
'fight' is the definition.
(I know this)
'sister's plan' is the wordplay.
'plan' indicates anagramming the letters (plan can mean organise).
'sister' with letters rearranged gives 'RESIST'.
(Other definitions for resist that I've seen before include "Withstand or defy" , "Endure" , "Go against" , "Defy for sister" , "Withstand a force" .)