Vagrant stealing first part of investigator's brief (7)
I believe the answer is:
dossier
'brief' is the definition.
(both can mean a collection of information)
'vagrant stealing first part of investigator's' is the wordplay.
'vagrant' becomes 'dosser' (informal term for a vagrant).
'stealing' indicates putting letters inside.
'first part of' suggests taking the first letters.
The first letter of 'investigators' is 'i'.
'dosser' placed around 'i' is 'DOSSIER'.
(Other definitions for dossier that I've seen before include "Record" , "A collection of papers" , "File of papers (sometimes dodgy)" , "Collection of documents on a topic" , "Set of papers" .)