As an alternative to what Alfred mentioned, you can use the Vector Crop Tool to hide a part of the shape. It is non-destructive, which means you can go back later with the same tool & change how much of the shape is hidden. It only offers rectilinear cropping, so it is less useful for irregularly shaped objects but for squares & rectangles it is fast & easy.
If you created the square with the Rectangle Tool, the first thing you need to do is convert it to curves. Now choose the Node Tool so that you get 'Break curve', 'Close curve' and 'Join curves' buttons in the 'Action' section of the Context toolbar. Select a node at one end of the side you want, click on 'Break curve' and repeat with the node at the other end. Press the Esc key to deselect the square, click on the detached line to reselect it and then press Delete to remove it.
When you have two open paths selected, 'Join curves' will join them at the nearest nodes. Click on the 'Close curve' button to close the new curve.
