First, start up the Preview app.
Go into iTunes and bring up the song. If need be, click on the bar in the left-hand column to make it show Selected Song.
Ctrl-click on the art and copy.
Go over to Preview and File->New to get your cover displayed.
Move the cursor to one corner of the border you want, and shift-drag to select (highlight) the part of the image you want to keep. Edit->Copy to move the selection to the clipboard.
Back to iTunes, ctrl-click on the artwork pane again and pick paste.
That might appear to do nothing, so there is one last step. File->Get info on the track, and click the artwork tab. You can either drag your cropped art to the left of the well to make it the visible one, or you can delete the artwork you no longer want.