This can't be right (5)
I believe the answer is:
error
This might be a double definition.
'this can't' is the first definition.
The definition suggests a singular noun which matches the answer.
'right' is the second definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'be' is the link.
Can you help me to learn more?
(Other definitions for error that I've seen before include "Boob" , "Bug" , "Solecism" , "1 down" , "Bloomer, blunder" .)