Nobody else will know if we keep it (6)
I believe the answer is:
secret
'it' is the definition.
The definition suggests a singular noun which matches the answer.
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for secret that I've seen before include "One not to be divulged" , "Unidentified" , "Not generally known" , "Private, covert" , "Not seen or know by others" .)