Let ___ a secret

The answer to the crossword question “Let ___ a secret is INON” is “in on“. The phrase “in on” means to be aware of or knowledgeable about a secret or hidden information. Therefore, to “let in on a secret” means to reveal a secret to someone who was not previously aware of it. In the context of the crossword question, the missing word is “in” and completing the phrase with “on” creates the phrase “in on“.