The most common and most likely answer to this crossword clue is the 7 letter word UNCLOAK
I've seen this crossword clue in Independent
U
N
C
L
O
A
K
More clues leading to the result UNCLOAK
Buddy explains!

The straight part
We know that "13" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case 13 = uncloak.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 UNCLOAK mean?
UNCLOAK verb- To remove a cloak or cover from; to deprive of a cloak or cover; to unmask; to reveal.
- To remove one's cloak.
- To become visible again by turning off a cloaking device.