The most common and most likely answer to this crossword clue is the 9 letter word HARMONIZE
H
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The straight part
We know that "Agree" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case agree = harmonize.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 HARMONIZE mean?
HARMONIZE verb- To be in harmonious agreement.
- To play or sing in harmony.
- To provide parts to.
- To bring things into harmony, or to make things compatible.
- To provide the harmony for a melody.