Plain case holding papers (7)
I believe the answer is:
evident
'plain' is the definition.
(I know that plain can be written as evident)
'case holding papers' is the wordplay.
'case' becomes 'event' (case can mean an event or occurence).
'holding' means one lot of letters goes inside another.
'papers' becomes 'ID' (identity papers).
'event' enclosing 'id' is 'EVIDENT'.
(Other definitions for evident that I've seen before include "Noticeable" , "Clearly discernible" , "on display" , "Patent" , "Indisputable" .)