The most common and most likely answer to this crossword clue is the 14 letter word CORRESPONDENCE
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The straight part
We know that "Agreement/writing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case agreement/writing = correspondence.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 CORRESPONDENCE mean?
CORRESPONDENCE noun- Friendly discussion.
- Reciprocal exchange of civilities, especially conversation between persons by means of letters.
- An agreement of situations or objects with an expected outcome.
- Newspaper or news stories, generally.
- A postal or other written communication.
- Postal or other written communications.
- A relation.
- According to Swedenborg, a relationship of similarity between physical and spiritual things, such as that of light to wisdom, or warmth to love.