Plain case containing papers (7)
I believe the answer is:
evident
'plain' is the definition.
(I know that plain can be written as evident)
'case containing papers' is the wordplay.
'case' becomes 'event' (synonyms).
'containing' is an insertion indicator.
'papers' becomes 'ID' (identification papers).
'event' enclosing 'id' is 'EVIDENT'.
(Other definitions for evident that I've seen before include "Visible - plain" , "Noticeable" , "Clearly apparent" , "Clear to see" , "Incontrovertible" .)