Perhaps this is a stupid question, but I did not find the information I needed at the moment (maybe I just did not notice this option (???)).
How to clean or “crop” the area around the canvas?
I am very glad that the program allows you to leave the paint outside the canvas, it helped me more than once. But sometimes, in particular when transforming an object, this is very disturbing. I’ve given an example below…
Maybe the program has a cleaning function or the ability to edit the area behind the canvas?
Image → Trim to Image Size
The off canvas content will be removed and you’ll never see it again.
In Settings → Configure Krita → General → File Handling, there is an option, ‘Trim files before saving’ which will do this automatically if you really want to not have off-canvas content in saved .kra files.
For what I already experimented, just take care this doesn’t apply for layers that are locked 
Grum999
A good point and I wasn’t aware of that.
Does the Settings option have effect on layers that are locked?
I don’t know, didn’t tested: this option is too dangerous and I don’t activate it, too risky to forgot to deactivate it 
Grum999
My entire configs and resources are a radioactive mess so I don’t mind performing dangerous experiments
A locked layer does have off-canvas content removed when the image is saved if that option is enabled in the Settings.
My dev computer is a mess too, but as it’s a strong computer I use it often for heavy computation things (filter layers, styles, animation renders, …) and then this options is no way on my side; I used it once and erk… it was a disaster.
Good to know, thanks!
This is really the don’t touch nuclear option 
Grum999
Just to add: If you want only a certain layer to be affected, you can use the crop tool:
Make sure, that ‘Grow’ is deselected and ‘Applies to: Layer’ selected. Drag the handles all the way out to the canvas boundaries and hit ‘Crop’. Or you can leave some amount of off-canvas content intact by activating ‘Grow’ and drag the rectangle a bit outwards.
Thanks a lot for your help! I didn’t really notice the feature. Especially thanks for the tips they are helpful! 
@AhabGreybeard @Grum999 @dreamkeeper