The most common and most likely answer to this crossword clue is the 11 letter word CONCORDANCE
I've seen this crossword clue in The Times
C
O
N
C
O
R
D
A
N
C
E
Buddy explains!
The straight part
We know that "agreement" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case agreement = concordance.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 CONCORDANCE mean?
CONCORDANCE noun- Agreement; accordance; consonance.
- (grammar) Agreement of words with one another; concord.
- An alphabetical verbal index showing the places in the text of a book where each principal word may be found, with its immediate context in each place.
- A list of occurrences of a word or phrase from a corpus, with the immediate context.