How to fade to black without losing audio?

Using FCP X. I'm trying to fade to black at the end of a music video. I'm using audio from one clip, but I want the end of the video to be a fade to black from one clip with an audio fade from another clip. I added a slot of black at the end, and I created a transition to black with the video clip I want to fade, but the audio (which is from another clip) just ends abruptly when the transition starts. Please help.