Answer:
The meaning of the idiom "red herring" is something that is intentionally used to divert attention or mislead people from the main issue or topic being discussed.
Step-by-step explanation:
In this context, the mystery novelist introduced a character who was an ex-convict and made them a suspect in the story, even though they were later proven to be innocent. The purpose of this was to divert the reader's attention from the actual culprit and create suspense and intrigue in the plot.