The most common and most likely answer to this crossword clue is the 7 letter word SINNING
I've seen this crossword clue in The Daily Telegraph
S
I
N
N
I
N
G
More clues leading to the result SINNING
Buddy explains!
The straight part
We know that "being a naughty girl" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case being a naughty girl = sinning.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.
What does SINNING mean?
SINNING verb- To commit a sin.
- The act of committing a sin.