Possible Clues:

• Add nine and it all may have to be taken out
• Something added or removed?
• It comes last, and has no known use
• In the end in volume one may have to have it out
• Add-on
• The organ needs something added
• If a priest were to finish before nine, he might have to be taken out
• Does it take one priest to kill nine more? One may have to have it out with him
• Writer's addition - may be taken out in the theatre!
• So might a priest finish nine and have it out with them