Life-changing rasp (4)
I believe the answer is:
file
'rasp' is the definition.
(I've seen this in another clue)
'life-changing' is the wordplay.
'changing' indicates an anagram.
'life' anagrammed gives 'FILE'.
(Other definitions for file that I've seen before include "Line on board" , "Folder; rasp" , "Dossier; line of people" , "Document folder" , "(Set of) documents- line" .)