$ sudo apt-get update
$ sudo apt-get install cairo-dock-clock-plug-in 

cairo-dock-clock-plug-in is installed in your system.

Make ensure the cairo-dock-clock-plug-in package were installed using the commands given below,

$ sudo dpkg-query -l | grep cairo-dock-clock-plug-in *

You will get with cairo-dock-clock-plug-in package name, version, architecture and description in a table

cairo-dock-clock-plug-in package basic information: