The most common and most likely answer to this crossword clue is the 9 letter word FACTORIAL
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More clues leading to the result FACTORIAL
Buddy explains!

The straight part
We know that "!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ! = factorial.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 FACTORIAL mean?
FACTORIAL noun- The result of multiplying a given number of consecutive integers from 1 to the given number. In equations, it is symbolized by an exclamation mark (!). For example, 5! = 1 × 2 × 3 × 4 × 5 = 120.
- Of or pertaining to a factor or factorial.
- Of or pertaining to a factor, a kind of business agent.
- Of or pertaining to a factory.