The most common and most likely answer to this crossword clue is the 2 plus 3 plus 6 letter word OF ONE ACCORD
I've seen this crossword clue in The Times
O
F
O
N
E
A
C
C
O
R
D
Buddy explains!
The straight part
We know that "in agreement" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case in agreement = of one accord.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.