Hi Greg,
The clip needs a “point” at which it connects to the visual or video clip. There should be a point (usually at the beginning of the clip by default) where you can see the place where the audio connects to the separate video clip above. As Jeff says, try moving the audio clip away from the clip you are about to delete, so that the audio clip is connected to another video clip while you delete the other. Then you can move it back into position.
Alternatively, before deleting the video clip, simple select the audio clip, hit “Copy” or CMD and C to copy. Then once you have deleted the video clip, you simple hit “Paste” or CMD and V, and your audio clip will be copied back in.
The Seanacus