ClamAV is really only used to check for cross virus contamination. It's a tool that checks for windows malware inside of Linux.
Linux doesn't need any malware software. The way Linux runs and works is already way more secure in itself, almost everything you'll ever download is pre compiled intro software repositories that are checked constantly.
The only way you'll catch a virus on Linux is being dumb and clicking ads or downloading something from untrusted sources like websites that could be fake but look real.
What don't you completely understand about Linux firewall? I don't mind helping you learn