First of all, subclass.h is missing define guards and the class declaration is missing a semi-colon (. Furthermore, you do:
connect( pushButton_clear, SIGNAL( customSignal (const QString&) ), this, SLOT( mySlot(const QString&) ) );
connect( pushButton_clear, SIGNAL( customSignal (const QString&) ), this, SLOT( mySlot(const QString&) ) );
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Which doesn't compile either because pushButton_clear is a private member of the base class, thus inaccessible from the subclass. However, customSignal() is not a signal of the button anyway so you should change it to "this". Last but not least, you never create an instance of the subclass.
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