Well, this is just the opposite (3)
I believe the answer is:
ill
'well this is just the opposite' is the definition.
The definition suggests a singular noun which matches the answer.
This is all the clue.
Can you help me to learn more?
(Other definitions for ill that I've seen before include "Ailing" , "weak" , "Unhealthy" , "Infirm" , "Not well, sick" .)