The most common and most likely answer to this crossword clue is the 7 letter word UNLUCKY
I've seen this crossword clue in Financial Times
U
N
L
U
C
K
Y
More clues leading to the result UNLUCKY
- A French girl covering kilometres without a break
- A French king snared by girl without fortune
- One French girl covering kilometres without a break
- A French girl covering weekend as 13, supposedly
- Ill-omened, unlike a Kingsley Amis character?
- Tough peacekeepers, brave
- Inauspicious Brave quietly going after Ute and Navajo leaders
Buddy explains!

The straight part
We know that "as 13, supposedly" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case as 13, supposedly = unlucky.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 UNLUCKY mean?
UNLUCKY adjective- Unfortunate, marked by misfortune.
- Inauspicious.
- Having ill luck.
- Bringing ill luck.