Having added a couple of pages, finished it off (8)

'off' is the definition.
The answer and definition are different parts of speech. However, adjectives and past participle verbs can sometimes mean the same thing.

'having added a couple of pages finished it' is the wordplay.
I cannot really understand how this works, but
'a' is found in the answer.
'pages' could be 'pp' (an abbreviation for page written twice) and 'pp' is located in the answer.
'finished' could be 'ended' ('end' can be a synonym of 'finish') and 'ended' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?