Common knowledge (3-4)
I believe the answer is:
low-down
'common knowledge' is the definition.
Both the answer and definition are singular nouns.
Maybe you can see an association between them that I can't see?
'common knowledge' is the wordplay.
'common' becomes 'low' (both can mean vulgar).
'knowledge' becomes 'down' (I can't explain this - if you can you should believe this answer much more).
'low'+'down'='LOW-DOWN'
Can you help me to learn more?
(Other definitions for low-down that I've seen before include "Inside story" , "Info (slang) - dishonourable" , "Data - base" , "Mean, unfair" , "Inside information - mean (7;3-4)" .)