Life-changing document (4)
I believe the answer is:
file
'document' is the definition.
(thesaurus)
'life-changing' is the wordplay.
'changing' indicates anagramming the letters.
'life' with letters rearranged gives 'FILE'.
(Other definitions for file that I've seen before include "Folder; line" , "Document folder" , "Three Dn and this can indicate ordinary people" , "(Set of) documents- line" , "Smoothing tool - orderly batch of documents" .)