Do you use ClamAV as an antivirus on Trisquel?

No antivirus. Stick to trusted repositories of software (ideally only Trisquel's), never input your password if asked in a dubious context, and you should be fine.

I haven't used any antivirus in a very long time. Even when I was on Windows I didn't. Common sense and good browser extensions are my safety net. I am careful where I go and what I try to click on.

>"but I noticed that ClamAV is not updated in the Trisquel repository"

The version in the repo should be fine. Clam updates its virus definitions when you run it, so you'll still be getting up-to-date virus scanning. I use clam sometimes, but I haven't found any viruses that would affect a GNU/Linux installation. But you will occasionally see a Windows virus or trojan if people are sending you a lot of office documents or multi-media files. Maybe about once every 5 years or so. It's good to get rid of those so you don't inadvertently forward them on to someone that's using a Windows system.