Since this word is made up of a single kanji, it uses the kun'yomi reading. When learning the kanji, you didn't learn that reading, so here's a mnemonic to help you with this word:
There's a sound coming from inside the wall. It sounds like a knock, knock, knock noise. You break down the wall, and there's some creepy dude standing inside. He is the knocker (なか).
Let's go inside quickly.
The air conditioning was working well in the house.
The inside of the vacuum cleaner was empty.