Possible Clues:

• Whodunit, for example
• Whodunnit? It remains a secret
• Setter's almost back with your first puzzle
• Something difficult to understand
• Secretive person decapitated and buried in my tale of suspense
• Unexplained happening
• Something to figure out
• Carolyn Keene's genre
• Month without a set larking on railway? It can't be explained
• It's meaning to be hidden