Okay kiddo, so a clandestine church is a hidden church that people go to secretly. It's called clandestine because it's done in secret, like a secret club.
Sometimes people have to do this because they aren't allowed to practice their religion openly. Maybe the government or other people don't like their religion and try to stop them from practicing it. So they have to meet in secret to worship together.
It's kind of like a game of hide and seek. The people have to hide their church and make sure nobody finds it. And then they secretly invite people to come and worship with them.
It's not always easy to have a clandestine church because if someone finds out, they could get in trouble or even be punished. So it's important for the people to keep it a secret and only invite people they trust.
But they still want to practice their religion and worship together, so they do it in secret at a clandestine church.