The most common and most likely answer to this crossword clue is the 10 letter word NEGOTIATES
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Buddy explains!
The straight part
We know that "acts as diplomat" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case acts as diplomat = negotiates.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 NEGOTIATES mean?
NEGOTIATES verb- To confer with others in order to come to terms or reach an agreement.
- To arrange or settle something by mutual agreement.
- To succeed in coping with, or getting over something.
- To transfer to another person with all the rights of the original holder; to pass, as a bill.
- To transact business; to carry on trade.
- To intrigue; to scheme.