There's no telling if this is to be kept (6)
I believe the answer is:
secret
'there's no telling if this is to be kept' 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 secret that I've seen before include "Unidentified" , "Not seen or know by others" , "Concealed" , "Security classification level" , "Cloak and dagger" .)