You have two options, you can shutdown your Linux virtual machine and open your Documents > Virtual Machines folder and use the Finder to make a copy of your Linux VM bundle. Rename the clone to whatever name you want, then double-click it to open it in the Fusion library, when you power the VM on and Fusion asks did you "copy or move" the VM, choose copy.
Or go through the new machine wizard with a custom option and choose an existing virtual disk, open the virtual disk from the Linux VM you want to clone and Fusion will duplicate just the virtual disk for the new machine you're creating.
