A toxic aim needs changing — that’s self-evident (9)
I believe the answer is:
axiomatic
'self-evident' is the definition.
(I've seen this before)
'a toxic aim needs changing' is the wordplay.
'needs changing' indicates an anagram (I've seen this in other clues).
'toxic'+'aim'='toxicaim'
'toxicaim' is an anagram of 'xiomatic'.
'a'+'xiomatic'='AXIOMATIC'
'that's' acts as a link.
(Other definitions for axiomatic that I've seen before include "Accepted" , "Given" , "Self-evidently true" , "Taken for granted" , "Pertaining to self-evident truths" .)