To run single C/C++ file, you need install C/C++ Single File Execution plugin in CLion Preferences - Plugins.
When first time you open this project, if cmake-build-debug directory not exists, you may need to go to File menu
and click Reload CMake Project option to make CLion create cmake-build-debug directory.
Once you add a new C/C++ file, right click on the new C/C++ file editor, click the add executable for single c/cpp file
option, then you can run this single C/C++ file.