The most common and most likely answer to this crossword clue is the 5 plus 2 plus 3 plus 3 letter word QUEEN OF THE MAY
I've seen this crossword clue in The Guardian
Q
U
E
E
N
O
F
T
H
E
M
A
Y
Buddy explains!
The straight part
We know that "Girl at festival" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case girl at festival = queen of the may.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.