Solution

This is a topic I also used to struggle on. But luckily, I found a way to do this. First, make the FrameClass inherit the WindowClass. Next, in the __init__ function of the FrameClass, initialize the WindowClass. Now, when you want to access an attribute of FrameClass in the WindowClass use self. This works because when you initialize the WindowClass in the FrameClass, the FrameClass passes its object (In the code below it is 'obj') into the WindowClass (self refers to the object through which the method was called). Next, if you want to access an attribute of the WindowClass in the FrameClass, you can either use self or super. super makes things faster but can only be used for methods and self searches in the MRO(First child class, then parent class) and can be used for attributes too. Here is your code to make things simpler to understand:

Code

import tkinter as tk

class WindowClass:
    def get_value(self):
        value = self.entrybox.get()
        print(value)
        "DO WHATEVER YOU WANT TO DO WITH THE VALUE HERE"

    def __init__(self):
        root = tk.Tk()
        self.master = root
        self.master.geometry('500x500')

        self.create_frame_button = tk.Button(self.master,
                                             text='Create Frame with Entry',
                                             width=20,
                                             command=lambda: self.create_frame(self.master))

        self.get_value_button = tk.Button(self.master,
                                          text='Get value from frame',
                                          width=20,
                                          command=self.get_value)

        self.create_frame_button.place(x=10, y=10)
        self.get_value_button.place(x=10, y=40)

        root.mainloop()

class FrameClass(WindowClass):
    def __init__(self):
        super().__init__()

    def create_frame(self, master):
        self.__master = master

        self.frame = tk.Frame(self.__master, height=250, width=480, relief='solid', bd=4)
        self.entrybox = tk.Entry(self.frame, width=15, font='Calibri, 12')

        self.entrybox.place(x=10, y=10)
        self.frame.place(x=10, y=100)

if __name__ == '__main__':
    obj = FrameClass()

Note: It is usually recommended to create the GUI in another function not in __init__. I have removed the create_frame function and replaced it with a lambda function. I have also used name mangling to avoid confusion between the self.master of both the classes. I hope this helped you to solve your problem and increased your knowledge of OO Tkinter! Also, if you couldn't understand anything about the information given in the answer, you can comment and ask.