The most common and most likely answer to this crossword clue is the 9 letter word CO-AUTHOR
I've seen this crossword clue in The Guardian
C
O
-
A
U
T
H
O
R
Buddy explains!
The straight part
We know that "One’s not self-sufficient" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one’s not self-sufficient = co-author.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 CO-AUTHOR mean?
CO-AUTHOR noun- An author who collaborates with another to write something.
- To write something in collaboration with another author.