Life-changing tool (4)
I believe the answer is:
file
'tool' is the definition.
(file is a kind of tool)
'life-changing' is the wordplay.
'changing' indicates an anagram.
'life' anagrammed gives 'FILE'.
(Other definitions for file that I've seen before include "Dosssier" , "Tool for sawing through metal" , "Cutting tool or line of people" , "Set of related records kept together" , "(Set of) documents- line" .)