How well kept they are, nobody knows (7)

I believe the answer is:

secrets

I'm a little stuck... Click here to teach me more about this clue!

'how well kept they' is the definition.
The definition suggests a plural noun which matches the answer.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for secrets that I've seen before include "Undivulged facts" , "Confidential matters" , "They're not to be disclosed to others" , "Confidential data" , "Hidden facts" .)

I've seen this clue in the Evening Standard.

Want a hint initially instead of a full solution? Install my app