The most common and most likely answer to this crossword clue is the 10 plus 5 letter word HISTORICAL NOVEL
I've seen this crossword clue in The Daily Telegraph
H
I
S
T
O
R
I
C
A
L
N
O
V
E
L
More clues leading to the result HISTORICAL NOVEL
Buddy explains!
The straight part
We know that "‘The Scarlet Letter’, say" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ‘the scarlet letter’, say = historical novel.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.