Clue in lawsuit that barrister brings to trial? (9)
I believe the answer is:
briefcase
'that barrister brings to trial?' is the definition.
'clue in lawsuit' is the wordplay.
'clue in' becomes 'brief' (both can mean to inform someone about something).
'lawsuit' becomes 'case' ('case' can be a synonym of 'lawsuit').
'brief'+'case'='BRIEFCASE'
(Other definitions for briefcase that I've seen before include "Carried by lawyer" , "One may hold papers" , "providing security for the papers" , "See fabric covering container for business papers" , "Portable receptacle for documents" .)