xfce4-clipman-plugin from home:manfred-h:X11:xfce project
Select Your Operating System
Add repository and install manually
For openSUSE Leap 15.1 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:manfred-h:X11:xfce/openSUSE_Leap_15.1/home:manfred-h:X11:xfce.repo zypper refresh zypper install xfce4-clipman-plugin