One who's opposed ain't going to change (4)
I believe the answer is:
anti
'one' is the definition.
Both the answer and definition are adjectives. Perhaps you can see an association between them that I can't see?
'opposed ain't going to change' is the wordplay.
'ain't' anagrams to 'ANTI'.
However, I'm not sure how this is indicated.
'who's' acts as a link.
Can you help me to learn more?
(Other definitions for anti that I've seen before include "Who pooh-poohs all" , "Adversary" , "An opponent - opposed to party policy, etc" , "Someone opposed (to a policy?)" , "Opposition member" .)