The most common and most likely answer to this crossword clue is the 1 plus 5 letter word O LEVEL
O
L
E
V
E
L
Buddy explains!
The straight part
We know that "an old UK school qualification" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case an old uk school qualification = o level.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.