import tkinter as tk
from tkinter import ttk

def combo_changed(_varname, _index, _operation):
    print('changed')
    reset()

# main window
window = tk.Tk()

# main frame
frame = ttk.Frame(window)
frame.grid()

# combobox
cb = ttk.Combobox(frame, values=['1','2'])
cb.grid()

# entry
entry = ttk.Entry(frame)
entry.grid()

# combobox var and event
tk_var = tk.StringVar()
cb.configure(textvariable=tk_var)
trace_id = tk_var.trace_add('write', combo_changed)

def reset():
    tk_var.trace_remove('write', trace_id)
    cb.grid_forget()
    cb.destroy()