Left a sheet in one's briefcase (9)
I believe the answer is:
portfolio
'one's briefcase' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe there's an association between them I don't understand?
'left a sheet' is the wordplay.
'left' becomes 'port' (left side of a ship - opposite of starboard).
'a sheet' becomes 'folio' (folio is a kind of sheet).
'port'+'folio'='PORTFOLIO'
'in' is the link.
Can you help me to learn more?
(Other definitions for portfolio that I've seen before include "Set of collected pieces of creative work" , "Large case" , "Collection of papers" , "artist's sample work" , "responsibility of minister" .)