Mark papers to be returned by a reviewer (9)
I believe the answer is:
diacritic
'mark' is the definition.
(diacritic is a kind of mark)
'papers to be returned by a reviewer' is the wordplay.
'papers' becomes 'id' (identification papers).
'to be returned' says the letters should be written backwards (I've seen 'is returned' mean this).
'by' is a charade indicator (letters next to each other).
'reviewer' becomes 'critic' (I've seen this before).
'id' written backwards gives 'di'.
'di'+'a'+'critic'='DIACRITIC'
(Other definitions for diacritic that I've seen before include "accent, for example" , "Mark" , "one showing a pronounced difference" , "[ACCENT] , perhaps" , "Capable of distinguishing" .)