A fence I’d altered leading to opposition (8)
I believe the answer is:
defiance
'opposition' is the definition.
(defy can mean to oppose or resist)
'a fence i'd altered' is the wordplay.
'altered' indicates an anagram.
'a'+'fence'+'id'='afenceid'
'afenceid' is an anagram of 'DEFIANCE'.
'leading to' is the link.
(Other definitions for defiance that I've seen before include "Bold disobedience" , "Disregard for authority" , "Refusal to comply" , "Challenging behaviour" , "Challenging attitude" .)