The most common and most likely answer to this crossword clue is the 10 letter word VALIDATION
I've seen this crossword clue in The Guardian
V
A
L
I
D
A
T
I
O
N
Buddy explains!
The straight part
We know that "Proof" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case proof = validation.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 VALIDATION mean?
VALIDATION noun- The act of validating something.
- Something, such as a certificate, that validates something; attestation, authentication, confirmation, proof or verification.
- The process whereby others confirm the validity of one's emotions.
- The process of identifying a new prisoner's gang affiliation.