The most common and most likely answer to this crossword clue is the 1 plus 5 plus 5 letter word A CLEAN SLATE
I've seen this crossword clue in The Daily Telegraph
A
C
L
E
A
N
S
L
A
T
E
More clues leading to the result A CLEAN SLATE
Buddy explains!
The straight part
We know that "A fresh start for pupils before they were eleven" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a fresh start for pupils before they were eleven = a clean slate.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.