Rightclick on clip in Project window, modify: break out to mono.
sorry I'm not sure I follow. right click on a clip (in the timeline/project monitor) doesn't give any - "modify:break out to mono"
Instead of Rt-clicking on the Clip in the Timeline, Delete that, and then Rt-click on the Clip in the Project Panel, before dragging to the Timeline.
Sorry I found it
thank you very much