Drag & drop the clip on timeline. Click on edit menu & click on detach audio.
Still you can see audio and video will have a little connecting link to remove it.
Click on detach track icon (on the left side of the audio track.
Now we can freely move audio part anywhere.
Now click on video on timeline and hit delete key (optional step).
To save this audio as mp3 click on export menu.
Click on export audio. By default MP3 option is selected.
Give it a title & click on start button.

Go to the folder where you saved it & play it to check