For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:manfred-h:X11:xfce:4.20/openSUSE_Tumbleweed/home:manfred-h:X11:xfce:4.20.repo
zypper refresh
zypper install xfce4-clipman-plugin
For openSUSE Slowroll run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:manfred-h:X11:xfce:4.20/openSUSE_Slowroll/home:manfred-h:X11:xfce:4.20.repo
zypper refresh
zypper install xfce4-clipman-plugin
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:manfred-h:X11:xfce:4.20/15.6/home:manfred-h:X11:xfce:4.20.repo
zypper refresh
zypper install xfce4-clipman-plugin