Property:
clearRect:

Canvas:
your browser does not support the canvas tag
Code:
var c = document.getElementById('myCanvas');
var ctx = c.getContext('2d');
ctx.fillStyle = 'red';
ctx.fillRect(0,0,300,150);
ctx.clearRect(20,20,100,50);
Click the property values above to see the result
W3Schools.com - Play it