Quoted from ClamAssassin's README file:

Run './configure' which will try to figure out where things are installed
based on your path.  It will use clamdscan instead of clamscan if it sees
/tmp/clamd.  Make sure to read the notes below on using clamdscan to make
sure your system is configured properly.  If you want to force clamdscan to
be used, or force it not to be used, you can try the following options to
configure:

  --enable-clamdscan
        Force clamassassin to use clamdscan even if /tmp/clamd not found.
  --disable-clamdscan
        Force clamassassin not to use clamdscan even if /tmp/clamd found.

So, since /etc/rc.conf.local has the following,

clear_tmp_enable="YES"	# clear /tmp on reboot

when getmail, fetchmail, or whatever you've configured to get your
messages runs, there's not going to be a directory, /tmp/clamd, since
/tmp has been cleared on boot. So, you can add the following to
/etc/rc.local:

f [ -x /bin/mkdir ]; then
        /bin/mkdir /tmp/clamd && chown _clamav:_clamav /tmp/clamd
fi

That'll take care of clamassassin if you configured it to use clamd.