The most common and most likely answer to this crossword clue is the 12 letter word BACHELORETTE
I've seen this crossword clue in The Times
B
A
C
H
E
L
O
R
E
T
T
E
More clues leading to the result BACHELORETTE
Buddy explains!

The straight part
We know that "Unmarried lady" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case unmarried lady = bachelorette.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 BACHELORETTE mean?
BACHELORETTE noun- An unmarried woman.
- A very small bachelor (single room) apartment.