The most common and most likely answer to this crossword clue is the 1 plus 4 plus 2 plus 7 letter word A GAME OF THRONES
I've seen this crossword clue in Independent
A
G
A
M
E
O
F
T
H
R
O
N
E
S
Buddy explains!
The straight part
We know that "fantasy novel" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fantasy novel = a game of thrones.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.