If I have correctly understood what you are asking it seems you have an existing video whose audio you wish to use with another video. Is that correct?
If so, perhaps the simplest method is: open the video whose audio you want to use on the timeline and then go to Fil>Export movie>Audio as WAV. (You will be asked where you want to save the resulting audio file.) Once completed, close that video file, open the second video on the timeline and drag the wav file you've just created to a higher-numbered track.
Assuming from the title of the topic that the main clip is going to be cut from/to and there are other video clips you will be cross cutting to the simplest method is:
Place the full length video clip on track 1 - see example below
Put the other video clips on lower tracks and mute their audio, if you want the clips to crossfadde apply a fade in/out to these clips
If you only need the audio track from the main clip you can ungroup this from the video using the Ungroup button - the broken chain link you can see in the above image, the right button of the 2 between the highlighted buttons top left, and delete the video portion of the clip. For the other clips arrange as required on a lower track.
The more complex method assuming there are no more than 4 cameras is to use the Multicam edit mode - for more information on using this see the manual available under Help, About.