Noun and する verb
みっこく
とある会社の脱税について、税務署に密告したいと思ってるんです。
I’d like to inform the tax office about a company’s tax evasion.
In secrecy you must announce this information. It's not for anyone to hear, it's secret information.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own. The みつ goes to みっ.