My cron doesn't use CRON in the subject - is this a configuration you set in the crontab file?
The only way I have found is:
/path_to_script/script 2>&1 | mailx -s "Subject" userid@hostname
The scripts (or the crontab entry) I use have this for the command to run. Since there is no output from the command (both stdout and stderr are sent to mailx) no explicit mail is generated - leaving only that provided by the mailx command.
You can do this with:
$ at now
script.sh 2>&1 | mailx -s "CRON: whatever" user@server
Or just have it embedded in a very short script.