The most common and most likely answer to this crossword clue is the 8 letter word DIPLOMAS
I've seen this crossword clue in The Times
D
I
P
L
O
M
A
S
Buddy explains!
The straight part
We know that "Proof of competence?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case proof of competence? = diplomas.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 DIPLOMAS mean?
DIPLOMAS noun- A document issued by an educational institution testifying that the recipient has earned a degree or has successfully completed a particular course of study.