The most common and most likely answer to this crossword clue is the 7 letter word OUABAIN
O
U
A
B
A
I
N
Buddy explains!
The straight part
We know that "A killer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a killer = ouabain.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 OUABAIN mean?
OUABAIN noun- A poisonous cardiac glycoside, g-strophanthin, found in the seeds of certain lianas.