The most common and most likely answer to this crossword clue is the 9 letter word BLOODBATH
I've seen this crossword clue in The Guardian
B
L
O
O
D
B
A
T
H
Buddy explains!
The straight part
We know that "Massacre" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case massacre = bloodbath.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 BLOODBATH mean?
BLOODBATH noun- Indiscriminate killing or slaughter, a massacre.
- An aggressive or very violent contest or confrontation.
- An upset (as of a game with unexpected results, or a national presidential convention) or heavy defeat.
- A large financial loss or massive layoff brought about by negative economic conditions.
- A bath taken in warm blood used as a restorative or medical treatment.