For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/X11:xfce:4.18/openSUSE_Tumbleweed/X11:xfce:4.18.repo
zypper refresh
zypper install xfce4-clipman-plugin
For openSUSE Slowroll run the following as root:
zypper addrepo https://download.opensuse.org/repositories/X11:xfce:4.18/openSUSE_Slowroll/X11:xfce:4.18.repo
zypper refresh
zypper install xfce4-clipman-plugin
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/X11:xfce:4.18/15.6/X11:xfce:4.18.repo
zypper refresh
zypper install xfce4-clipman-plugin
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/X11:xfce:4.18/15.5/X11:xfce:4.18.repo
zypper refresh
zypper install xfce4-clipman-plugin