The most common and most likely answer to this crossword clue is the 9 letter word EVIDENCED
E
V
I
D
E
N
C
E
D
More clues leading to the result EVIDENCED
Buddy explains!

The straight part
We know that "it's proven" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case it's proven = evidenced.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 EVIDENCED mean?
EVIDENCED verb- To provide evidence for, or suggest the truth of.