import tkinter as tk

class App(tk.Frame):
    def __init__(self, master):
        tk.Frame.__init__(self, master, height=42, width=42)
        self.entry = tk.Entry(self)
        self.entry.focus()
        self.entry.pack()
        self.clear_button = tk.Button(self, text="Clear text", command=self.clear_text)
        self.clear_button.pack()

    def clear_text(self):
        self.entry.delete(0, 'end')

def main():
    root = tk.Tk()
    App(root).pack(expand=True, fill='both')
    root.mainloop()

if __name__ == "__main__":
    main()