The most common and most likely answer to this crossword clue is the 5 letter word CRONE
I've seen this crossword clue in The Guardian
C
R
O
N
E
More clues leading to the result CRONE
Buddy explains!

The straight part
We know that "Hag" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case hag = crone.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 CRONE mean?
CRONE noun- An old woman.
- An archetypal figure, a Wise Woman.
- An ugly, evil-looking, or frightening old woman; a hag.
- An old ewe.
- An old man, especially one who talks and acts like an old woman.