A clearance sacrifice is a type of sacrifice as its name suggests. You would use it when you have a chance to do a good move but a piece is in the way. If you move it away your opponent will gain valuable time. So you have to get this piece out of the way, probably by checking the opponent's king. After this you will not give him any time and it is out of your way. Here is a great example of this kind of sacrifice:Hope you enjoyed it and feel free to add comments and edit.