Added a couple of pages, which finished it (8)
I believe the answer is:
appended
'added' is the definition.
('append' can be a synonym of 'add')
'a couple of pages which finished it' is the wordplay.
I cannot quite understand how this works, but
'a' is found in the answer.
'pages' could be 'pp' (an abbreviation for page written twice) and 'pp' is present in the answer.
'finished' could be 'ended' (similar in meaning) and 'ended' is located in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for appended that I've seen is " Added to the end".)