Add a couple of pages to conclusion (6)
I believe the answer is:
append
'add' is the definition.
('append' can be a synonym of 'add')
'a couple of pages to conclusion' is the wordplay.
'couple of' says to double the letters.
'pages' becomes 'p'.
'to conclusion' becomes 'end' (end is a kind of conclusion).
'p' doubled is 'pp'.
'a'+'pp'+'end'='APPEND'
(Other definitions for append that I've seen before include "Add on, attach" , "Napped (anag)" , "Add (at the back of a document)" , "Add (to the bottom of a document)" , "Add, annex" .)