The most common and most likely answer to this crossword clue is the 8 plus 4 letter word MARGARET PAGE
I've seen this crossword clue in The Guardian
M
A
R
G
A
R
E
T
P
A
G
E
Buddy explains!
The straight part
We know that "one of 18 13 down 29 26 across" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one of 18 13 down 29 26 across = margaret page.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.