The most common and most likely answer to this crossword clue is the 4 plus 6 letter word TEST FLIGHT
I've seen this crossword clue in The Daily Telegraph
T
E
S
T
F
L
I
G
H
T
Buddy explains!
The straight part
We know that "that gives proof of high-level suitability" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case that gives proof of high-level suitability = test flight.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.