Tag on a couple of pages to finish (6)
I believe the answer is:
append
'tag on' is the definition.
(synonyms)
'a couple of pages to finish' is the wordplay.
'couple of pages' becomes 'PP' (the abbreviation for 'page' repeated).
'to finish' becomes 'end' (come to an end).
'a'+'pp'+'end'='APPEND'
(Other definitions for append that I've seen before include "provided as add-on" , "Hang on" , "Napped (anag)" , "Affix to the end" , "Add, annex" .)