I wouldn't clear anything in /tmp -- look elsewhere for low hanging fruit. Typical offenders are log files (if you haven't enabled logrotate on them), user's IMAP trash folders, DB backups, and even daily backups if users have implemented a CMS-level (WordPress) backup solution -- so you wind up with large compressed archives in triplicate: daily, weekly, monthly).
You could search for files 100MB or larger:
sudo find / -xdev -type f -size +100M
Use your judgement to selectively delete some XL size files. And then likely you could truncate some of the log files to 0 bytes and start fresh, along with implementing logrotate:
truncate -s0 /var/log/spamd.log