A writer Richard's mentioned added pages (8)
I believe the answer is:
appendix
'pages' is the definition.
(I have seen 'extra pages' mean 'appendix' so perhaps 'pages' could also mean 'appendix')
'a writer richard's mentioned added' is the wordplay.
I cannot quite see how this works, but
'a' is within the answer.
'writer' could be 'pen' (something which writes) and 'pen' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for appendix that I've seen before include "extra pages" , "Subsidiary section" , "Supplementary material at the end of a document" , "Extra bit" , "Supplementary material at the back" .)