Document carrier (9)
(Other definitions for briefcase that I've seen before include "See fabric covering container for business papers" , "Carrying container for files" , "providing security for the papers" , "One transfers papers" , "Carried by lawyer" .)
'document carrier' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for attache case that I've seen before include "Carrier for documents, etc" , "Document carrier" , "Small briefcase" , "Diplomatic incident" , "This will grip the papers" .)