Extras, after opener's dismissed, depress me (7)
I believe the answer is:
addenda
'extras' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for addenda that I've seen before include "Additions to documents" , "Pieces added to the end" , "Extras" , "supplement" , "Items added to a text" .)