The most common and most likely answer to this crossword clue is the 8 plus 9 letter word ALGERNON MONCRIEFF
I've seen this crossword clue in The Guardian
A
L
G
E
R
N
O
N
M
O
N
C
R
I
E
F
F
Buddy explains!
The straight part
We know that "26 character /26 character" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case 26 character /26 character = algernon moncrieff.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.