from tkinter import *
import time

class App(Frame):
    def __init__(self,master=None):
        Frame.__init__(self,master)
        self.master=master
        self.label=Label(text="",fg="Green",bg="Black",font=("Helvetica",18))
        self.label.place(x=50,y=80)
        self.update_clock()

    def update_clock(self):
        now=time.strftime("%H:%M:%S")
        self.label.configure(text=now)
        self.after(1000, self.update_clock)

root=Tk()
app=App(root)
root.wm_title("Tkinter clock")
root.geometry("200x200")
root.after(1000, app.update_clock)
root.mainloop()