The most common and most likely answer to this crossword clue is the 8 letter word KATHLEEN
I've seen this crossword clue in Independent
K
A
T
H
L
E
E
N
More clues leading to the result KATHLEEN
Buddy explains!

The straight part
We know that "Girl" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case girl = kathleen.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.