The most common and most likely answer to this crossword clue is the 9 letter word SOUNDNESS
S
O
U
N
D
N
E
S
S
More clues leading to the result SOUNDNESS
Buddy explains!

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 SOUNDNESS mean?
SOUNDNESS noun- The state or quality of being sound.
- The result or product of being sound.
- The property (of an argument) of not only being valid, but also of having true premises.
- The property of a logical theory that whenever a wff is a theorem then it must also be valid. Symbolically, letting T represent a theory within logic L, this can be represented as the property that whenever T \vdash \phi is true, then T \vDash \phi must also be true, for any wff φ of logic L.