Since you're running clamd from /usr/local/sbin, I assume that you have installed clamav from source. clamd is a binary that starts the clamd daemon, so it does not use the "start" option. Also as it's a daemon, it will run in background, so no need for "&" also.
What you can do is to put your command in rc.local, so it will be executed on system startup.
If you have used a rpm package you could use "service clamd start"