The most common and most likely answer to this crossword clue is the 5 letter word NELLY
I've seen this crossword clue in The Daily Telegraph
N
E
L
L
Y
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 = nelly.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 NELLY mean?
NELLY noun- A person's life.
- An effeminate homosexual man.
- A silly person.
- A giant petrel, Macronectes giganteus and Macronectes halli.
- Unmanly; effeminate.