There is no clear function in u8g library. You need to overwrite the old screen.
Or you can also set the color index to zero and draw a box the size of the screen. This clears everything that was displayed. You must return the color index to one in a separate page and you can go ahead and display something again. TFT LCDs are too difficult to handle that's why I always prefer
LCD 16x2.
u8g.setColorIndex(0);
u8g.drawBox(1, 1, 64, 128);