The most common and most likely answer to this crossword clue is the 11 letter word TRANSACTION
I've seen this crossword clue in Financial Times
T
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More clues leading to the result TRANSACTION
Buddy explains!

The straight part
We know that "deal" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case deal = transaction.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.
Crossword Answer definitions
TRANSACTION noun- The act of conducting or carrying out (business, negotiations, plans).
- A deal or business agreement.
- An exchange or trade, as of ideas, money, goods, etc.
- The transfer of funds into, out of, or from an account.
- An atomic operation; a message, data modification, or other procedure that is guaranteed to perform completely or not at all (e.g. a database transaction).
- (especially in plural) A record of the proceedings of a learned society.
- (in transactional analysis) A social interaction.