For a while it was possible to create effects that burned timecode onto clips, but due to a recent update from Apple, these effects don't work in Final Cut Pro X at the moment.

There is a workaround.

1. Open the clip in a timeline, add the Timecode generator
2. Select the clip in the event, create a new single-angle multicam clip based on the clip
3. Use the multicam clip in your projects
4. To turn off the timecode display, open the multicam clip in the angle viewer, then open the clip in its own timeline and delete or disable the timecode generator.

For more detail and screenshots, visit my blog.