Add a transition to the end of both the video and audio clips. The default duration is a half second. For an extended fade out, select the transition and drag the handle for the start to the left. You will likely need to zoom in so that you can see and grab the handles.
Note that if you're working with a video clip that has audio, the video and audio transitions are trimmed independently. To see the transition on the audio, click the Expand Audio button.