in 2.8" tft from adafruit, for clear screen, you can use this instruction....
you can use this instruction tft.clear(WHITE); but you need change the code in file Adafruit_ILI9341.cpp and Adafruit_ILI9341.h, Adafruit_GFX.cpp and Adafruit_GFX
This files are in libraries form adafruit, you need to download that libraries and put then in libraries from software arduino.
Link for the libraries for 2.8" tft from adafruit : https://learn.adafruit.com/adafruit-2-8-tft-touch-shield-v2/graphics-test
or you can use the original instruction without change the respective files.