Oppose unorthodox sister (6)
I believe the answer is:
resist
'oppose' is the definition.
(resisting is a kind of opposing)
'unorthodox sister' is the wordplay.
'unorthodox' indicates an anagram.
'sister' with letters rearranged gives 'RESIST'.
(Other definitions for resist that I've seen before include "Defy for sister" , "Offer opposition to" , "Forgo" , "Don't give in" , "Put up opposition to sister" .)