OK, let’s do it from scratch.
Boot into OS X.
1) Double check that you have 100% bootable clover version on your USB flash drive.
2) Mount USB drive EFI partition via Clover Configurator (CC) and copy EFI folder to Desktop.
Unmount USB EFI partition via CC.
View attachment 426748
3) Mount EFI partition from SSD via CC and delete all contents on it (Empty Trash).
Unmount EFI partition via CC.
View attachment 426754
4) Open EFI folder you’ve copied to your Desktop and inspect /Clover/Drivers/UEFI/ folder. Better to leave it open during next step.
5) Start Clover Bootloader installation. Select your NVMe SSD as target pressing Change Install Location. Next press Customise and select as many UEFI Drivers corresponding with your EFI folder from Desktop as you can then press install.
View attachment 426750
6) After installation is complete, copy all missing UEFI Drivers (if there are any) into /Clover/Drivers/UEFI/ folder on EFI SSD partition still mounted by Clover installer. Next synchronise all the folders in EFI SSD partition and EFI folder on your desktop except /Clover/Drivers/UEFI/ (it’s already done) and BOOT folder.
Congrats, you are done! Time to reboot and verify that your NUC able to boot without USB flash drive.
PS: use my screenshots just as references 'cause they from X99 mb, not from NUC