Hover your mouse button near the top edge of an event and the pointer will change into a small hand with a pointing finger. Drag down and a blue line will appear across the top of the clip. The farther you drag this line down, the less opaque (more transparent) the event will be.

You can also insert a Track Composite envelope. You can add nodes to this envelope to continuously vary the opacity from one point to the next across any events on that track.