The most common and most likely answer to this crossword clue is the 1 plus 1 plus 9 letter word H P LOVECRAFT
I've seen this crossword clue in Independent
H
P
L
O
V
E
C
R
A
F
T
Buddy explains!
The straight part
We know that "He wrote" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case he wrote = h p lovecraft.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.