Changing fate is an achievement (4)
I believe the answer is:
feat
'is an achievement' is the definition.
(I've seen this before)
'changing fate' is the wordplay.
'changing' is an anagram indicator.
'fate' anagrammed gives 'FEAT'.
(Other definitions for feat that I've seen before include "Deed; achievement" , "Achievement needing skill, strength, etc." , "Achievement needing great courage or skill" , "Notable achievement" , "Notable act" .)