There are many files could be stored in /tmp folder which are needed by running software on your Linux system. Removing them could cause you data loss.
However there is a safe way to do this.
Use the following command:
This will remove all files from tmp folder.
However there is more elegant solution, but possible less stable. Use TMPFS for /tmp folder. That means that everytime you reboot, you get clean /tmp folder, as all file are stored in RAM of your system.
To see how setup TMPFS follow this link: