Am first making changes to the final chapter (6)
I believe the answer is:
'am first' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'changes to the final chapter' is the wordplay.
'changes' becomes 'ing' (I can't justify this - if you can you should give a lot more credence to this answer).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'the final chapter' becomes 'end' (I've seen this in another clue).
'ing' after 'end' is 'ENDING'.
'making' is the link.
Can you help me to learn more?
(Other definitions for ending that I've seen before include "Gin den (anag.)" , "Closure" , "Climax" , "Armageddon?" , "Concluding" .)