Use the Clear/Load button in the Clip Library menu to delete the destination before loading the selected entry. For example, if you are loading a reel entry, the destination reel is deleted before the reel is loaded.
To clear the Desktop or a reel when you load clips:
The clips are loaded to the Desktop. If you loaded a Desktop, the clips that were previously on the Desktop are cleared and the selected clips are loaded to the Desktop. If you loaded a clip or reel, the reel you selected is cleared and the selected clips are loaded to a reel.