Add a couple of pages to 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 to finish' is the wordplay.
'couple of' says to double the letters.
'pages' becomes 'p'.
'to finish' becomes 'end' ('end' can be a synonym of 'finish').
'p' duplicated is 'pp'.
'a'+'pp'+'end'='APPEND'
(Other definitions for append that I've seen before include "Add (to the bottom of a document)" , "Affix to the end" , "provided as add-on" , "Add on at the bottom - say signature" , "Napped (anag)" .)