Add a couple of pages then finish (6)
I believe the answer is:
append
'add' is the definition.
(I know that add can be written as append)
'a couple of pages then finish' is the wordplay.
'couple of' indicates the doubling of some letters.
'pages' becomes 'p'.
'then' is a charade indicator (letters next to each other).
'finish' becomes 'end' (synonyms).
'p' duplicated is 'pp'.
'a'+'pp'+'end'='APPEND'
(Other definitions for append that I've seen before include "Adjoin" , "Affix to the end" , "provided as add-on" , "Add on, attach" , "Add, annex" .)