To ensure your system is up-to-date with the latest virus definitions from ClamAV, you would normally use "update" in the Antivirus Essentials Package, but that may no longer work. 

Why does it no longer work?

With Cisco taking stewardship of the ClamAV package, updates are now served over cloudflare (or similar) which blocks access from, well, automated tools like the updater. If you try and download the updates using wget or curl you'll receive HTML telling you access is denied, and this is what the freshclam tool sees when it tries. 

The use of some sort of web protection for updates is probably the most stupid idea of the year, but whatever, now you've got to fix it. 

Follow these steps to download and update the required files:

1. Stop the AntiVirus package using Package Manager. 

• Open a browser and download the following files.

    https://database.clamav.net/daily/cvd daily.cvd
    https://database.clamav.net/daily/cvdmain.cvd
    https://database.clamav.net/daily/cvdbytecode.cvd

• Upload these to your NAS in a shared folder. 
• Enable SSH access (if not already)
• SSH into the NAS.
• Upgrade with 

sudo su

• You will need to enter the administrator password here
• locate the files you just uploaded, for example if you uploaded them to a shared folder called TEMP, then temp may be found on /volume1/TEMP, but you might have to search about for it. Note where it is. 
• Update the files with

     cd /var/packages/AntiVirus/target/engine/clamav/var/lib
     rm *.cvd
     cp /path/to/download/*.cvd .

1. Restart the AntiVirus package using the Package Manager

• To check if the ClamAV definitions have been updated, open Antivirus Essentials and look at the last update date. You can now use ClamScan (aka Antivirus Essentials) as before. 

Remember to always back up important data before making major changes to a system.