The most common and most likely answer to this crossword clue is the 9 letter word PRECISION
I've seen this crossword clue in The Times
P
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More clues leading to the result PRECISION
Buddy explains!

The straight part
We know that "Exactitude" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case exactitude = precision.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 PRECISION mean?
PRECISION noun- The state of being precise or exact; exactness.
- The ability of a measurement to be reproduced consistently.
- The number of significant digits to which a value may be measured reliably.
- A bidding system that makes use of many artificial bids to describe a hand quite precisely.
- Used for exact or precise measurement.
- Made, or characterized by accuracy.