Being a cleaner in crime means that someone removes the traces of a bad thing that happened, like a mess or evidence that someone did something wrong. It's like when you spill a juice on the carpet, and your mom or dad cleans it up so it looks like it never happened.
But in crime, it's a bigger mess. For example, if someone breaks into a house and steals something, they might leave fingerprints, footprints, or even a piece of hair behind. This is evidence that could help the police figure out who did it. A cleaner comes in and gets rid of all of that evidence so that the police can't find out who did the bad thing.
It's not a good thing to be a cleaner in crime because it means that someone is helping bad people get away with doing bad things. It's important to always tell the truth and not hide things that are wrong.