The most common and most likely answer to this crossword clue is the 8 letter word APPENDIX
I've seen this crossword clue in The Daily Telegraph
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P
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More clues leading to the result APPENDIX
- A very soft goal upset team, in addition
- Part of book that may come out in the theatre
- A photographer's first photographs appearing around last supplement
- Arranged index on a couple of pages: it may be at back of book
- A very quiet finish before nine’s coda
- A two-page conclusion with one Times supplement?
- Reading matter a writer put on page ten in Le Monde
- Something that may be added or removed?
- Article with two pages to conclude one Times supplement
- Bit at end of book that might be taken out?
Buddy explains!

The straight part
We know that "Writer’s addition/that may be removed when it gets to the theatre" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case writer’s addition/that may be removed when it gets to the theatre = appendix.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does APPENDIX mean?
APPENDIX noun- Something attached to something else; an attachment or accompaniment.
- A text added to the end of a book or an article, containing additional information.
- The vermiform appendix, an inner organ that can become inflamed.
- Any process, prolongation or projection.