Means you actually know what’s going on, like you’ve done the reading, asked the right questions, or lived it. If you’re clued up on a topic, you can talk about it without waffling or chatting absolute rubbish. Often said with mild surprise, like, fair play, someone turned up prepared for once.
"Ask Priya about the tax return, she’s proper clued up. I tried doing mine and ended up crying into a Tesco meal deal while HMRC laughed somewhere in the distance"