Hello!
The virus definitions are update automatically. The new versions are distributed via ZCS version updates.
There is a post where a user shows how to update the clam av itself. I want to stress that this is an older post, and I don't know if it still works, but you can give it a shot if you wish.
Otherwise, we simply update the program/service in our updates.
[SOLVED] Howto: Update ClamAV