Note: Clip opacity is only enabled for Timeline Mode
To control clip opacity, enable "Show levels" under track settings
You will see a horizontal green line (known as rubber band) over your timeline clips:
You can drag the opacity control rubber band up or down to change the level.
Note: If no opacity nodes exist on the clip, the rubber band appears as a straight horizontal line across the entire clip.
Hover over your clip and click on rubber band to create an opacity node.
You can add as many opacity nodes as you like in your clip. Double click on a node to remove it.