The most common and most likely answer to this crossword clue is the 10 letter word PSEUDONYMS
I've seen this crossword clue in The Daily Telegraph
P
S
E
U
D
O
N
Y
M
S
More clues leading to the result PSEUDONYMS
Buddy explains!
The straight part
We know that "What writers sometimes assume" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case what writers sometimes assume = pseudonyms.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 PSEUDONYMS mean?
PSEUDONYMS noun- A fictitious name, as those used by writers and movie stars.