The most common and most likely answer to this crossword clue is the 6 plus 8 letter word BALLET MISTRESS
I've seen this crossword clue in The Daily Telegraph
B
A
L
L
E
T
M
I
S
T
R
E
S
S
Buddy explains!
The straight part
We know that "one of her pupils" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one of her pupils = ballet mistress.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.