Reading supplements (7)
I believe the answer is:
addenda
'reading supplements' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for addenda that I've seen before include "Codicils, riders or supplements" , "Items added to a text" , "Things added to the end of books" , "Postscripts, supplements" , "number of supplements" .)