For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:manfred-h:X11:xfce/openSUSE_Tumbleweed/home:manfred-h:X11:xfce.repo
zypper refresh
zypper install xfce4-clipman-plugin
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:manfred-h:X11:xfce/15.5/home:manfred-h:X11:xfce.repo
zypper refresh
zypper install xfce4-clipman-plugin
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:manfred-h:X11:xfce/15.4/home:manfred-h:X11:xfce.repo
zypper refresh
zypper install xfce4-clipman-plugin