Tetris Effect: Connected

A Tetris all clear is when your clear 4 lines at once, the only way you can do this is by putting putting a straight piece vertically down a gap that clears 4 of the rows.

You will get this achievement most likely over time, but if you're struggling try:
Effect Modes > All Clear

This game modes sets up pieces for you and some rounds sets up Tetris's for you so all you need to do is drop the pieces in.