I can't understand how Google search could fail. I didn't know the term either, so I entered »Kleereiber« als learned form the results that »Kleereiber« seems to be a synonym of »Kleereibe« (without ending -r) and that is must be a machine to clean grain. Then I entered »Kleereibe« and the very first two links are these:

1. You can buy a Kleereibe here: Website of a farm machine producer
2. Or here: Used Kleereiben on eBay

Other links lead to similar sites of to agricultural boards where people discussed where to buy and how to use these machines.

If you follow the first link, you will see some pictures, one of them even explains how the machine works, and there you also can read this:

... zum Lösen von Kleesamen aus den Blütenständen.

If you feed it into a good translator like deepl.com you will get this translation:

... for loosening clover seeds from the inflorescences.

So, a Kleereibe (fem.: die Kleereibe) or Kleereiber (masc.: der Kleereiber) is a machine to clean clover seed, but it can also be used to clear other grains. It is a cylinder of wire mesh and an moter-driven impeller that rubs the seed through the mesh. This is in a housing made of wood or metal.

I didn't know the term either, so I also googled for »Kleereiber« and learned form the results that »Kleereiber« seems to be a synonym of »Kleereibe« (without ending -r) and that is must be a machine to clean clover seeds. Then I entered »Kleereibe« and the very first two links are these:

1. You can buy a Kleereibe here: Website of a farm machine producer
2. Or here: Used Kleereiben on eBay

Other links lead to similar sites or to agricultural boards where people discussed where to buy and how to use these machines.

If you follow the first link, you will see some pictures, one of them even explains how the machine works:

And there you also can read this:

... zum Lösen von Kleesamen aus den Blütenständen.

If you feed it into a good translator like deepl.com you will get this translation:

... for loosening clover seeds from the inflorescences.

So, a Kleereibe (fem.: die Kleereibe) or Kleereiber (masc.: der Kleereiber) is a machine to clean clover seed, but it can also be used to clear other grains. It is a cylinder of wire mesh and an moter-driven impeller that rubs the seed through the mesh. This is in a housing made of wood or metal. Since it rubs clover through something, it is called »clover-rubber« which is »Kleereibe(r)« in German.