Best rebuttal (5)
I believe the answer is:
worst
This clue might be a double definition.
'best' is the first definition.
(I've seen this before)
'rebuttal' is the second definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
Can you help me to learn more?
(Other definitions for worst that I've seen before include "Most severe" , "Strangely, best" , "The least favorable" , "Of the poorest quality" , "Far from the best - defeat" .)