Life-changing dossier (4)
I believe the answer is:
file
'dossier' is the definition.
(a dossier is a file or set of information)
'life-changing' is the wordplay.
'changing' indicates an anagram.
'life' anagrammed gives 'FILE'.
(Other definitions for file that I've seen before include "Folder; line" , "Set of papers" , "Line of people, one behind the other" , "Set of related records kept together" , "Document folder" .)