How to add an EUR transaction for an USD supplier while having a completely different home currency?

Hello there, @Momedo.

You can add a supplier with foreign currency to track the transactions in the currency you pay with them. 

Please know that after you add a supplier and record transactions with them, you can't change the currency in their profile.

Here's how: 

1. Go to the Expenses tab from the left menu. 
2. Select Supplier, then New Supplier.
3. Enter the supplier's name and other info.
4. From the I pay this supplier with dropdown menu, select the currency(USD) you use to pay this vendor.
5. Click Save when you’re done.

Here's the link for a more detailed guide: Add a supplier with a foreign currency in QuickBooks Online.

Concerning the use of the journal entry in the clearing account, I'd suggest consulting an accountant for the proper posting of the transactions. 

For future reference, I've got you this link with some answers to commonly asked questions using the multi-currency feature in QuickBooks Online. 

I'll be here to guide you again if you have additional concerns in managing your foreign transactions. Take care!