Similar question has been in the Sketch Forum for half a year already, with no satisfying answer...
My clipboard is becoming an extremely messy merry-go-round. Isn't there even some kind of hack ?
And how can I delete cleared or empty drawings from a project (so not the whole project itself) ?
I can't copy or move drawings from one drawing to another, without turning it into a 'flat' image.
There is no way to clear the clipboard. It just has a fixed set of clippings that get recycled through. In addition to the merry-go-round view, there is also a grid view (tap the grid icon in the top left), which can be more convenient to look at if you have a lot of clippings. You can also choose to view just your "favorited" clippings instead of all your clippings (tap the star icon in the top right).
As for deleting drawings from a project, tap the edit button on a project (pencil icon in the top right of the project header), and tap the drawings you want to delete. Once some drawings are selected, tap the trash icon to delete those drawings. Be aware that this action is not undoable once you choose OK in the confirmation alert dialog.
Unfortunately, copying and moving drawings from one project to another is not supported. However, that and other organizer enhancements are on our roadmap.
Hope that helps,