The most common and most likely answer to this crossword clue is the 5 plus 8 letter word UPTON SINCLAIR
I've seen this crossword clue in The Guardian
U
P
T
O
N
S
I
N
C
L
A
I
R
Buddy explains!
The straight part
We know that "US writer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case us writer = upton sinclair.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.