Oppose one's eccentric sister (6)
I believe the answer is:
resist
'oppose one's' is the definition.
'resist' can be an answer for 'oppose' (resisting is a kind of opposing). I am unsure of the 'one's' bit.
'eccentric sister' is the wordplay.
'eccentric' indicates an anagram.
'sister' with letters rearranged gives 'RESIST'.
Can you help me to learn more?
(Other definitions for resist that I've seen before include "Don't give in" , "Put up opposition, defy" , "Stand up to, fight against" , "Refuse to give in to" , "Refuse to comply" .)