First of all.. Visit the ClamAV website for the latest definitions (you have to right click and choose save link as… on the menu) and save them in your home directory, or just open up a terminal and do this..
sudo mv daily.cvd main.cvd /var/lib/clamav
Which will move the newly downloaded files to the correct /var/lib/clamav directory.
Now enter your admin password & you should have completely up to date virus definitions :D
WARNING: This will only work if the ClamAV team use the same link every time for the daily updates?
To keep up to date with the definitions then, open a terminal &
Which will download the latest definitions to your home directory, then..
sudo mv daily.cvd /var/lib/clamav
..to transfer them to the correct directory again.
Just found this out myself and it should help you all out :)