A problem about "undefined reference to `Clean::Clean(QObject*)'

[wysota]

I believe Clean constructor is defined.

If you didn't define it in a separate file that you didn't post here then I believe the constructor is not defined, only declared. And based on the project file that you posted I believe main.cpp is your only C++ file (apart qmlapplicationviewer) thus I believe there are no other files in your project where you might have defined the constructor. This is basic C++ stuff, nothing Qt related.