Add a couple of pages before conclusion (6)
I believe the answer is:
append
'add' is the definition.
('append' can be a synonym of 'add')
'a couple of pages before conclusion' is the wordplay.
'couple of' shows that letters should be duplicated.
'pages' becomes 'p'.
'before' means one lot of letters go next to another.
'conclusion' becomes 'end' (end is a kind of conclusion**).
'p' duplicated is 'pp'.
'a'+'pp'+'end'='APPEND'
(Other definitions for append that I've seen before include "Add on at the bottom - say signature" , "Adjoin" , "Hang on" , "provided as add-on" , "Add on, attach" .)