The most common and most likely answer to this crossword clue is the 2 plus 1 plus 2 plus 7 letter word AM I MY BROTHER
I've seen this crossword clue in The Times
A
M
I
M
Y
B
R
O
T
H
E
R
Buddy explains!
The straight part
We know that "First Murderer’s speech" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case first murderer’s speech = am i my brother.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.