Newbie to X. Overwrite part of clip? I DID search. Can't find answer.

If the clip is a connected clip, to overwrite into it the clip needs to be in a secondary storyline. Select the clip or the group of clips and convert them to a storyline. Select the shelf of the storyline and place the playhead where you want to make the edit. Edit the replacement clip from the browser into the secondary storyline.

I am not quite sure about your problem.

Just leave that little clip on top and it will play back and export perfectly . . . you shouldn't need to do anything else.

If you do want to learn about extra storylines, this 3 minute tutorial should tell you everything you need to know.

https://www.youtube.com/watch?v=jE6MF818Xi4&t=147s

I'm with Ian. If I'm understanding your first screen shot correctly and the marker shows the "bad" frame, why not just move the "good" frame over to the right to cover the noisy frame? Hit the 'a' key to make sure you're in selection (aka "Arrow") mode and grab the good frame and move it over to the right. The stacking order designates that the highest image has priority, so it will display instead of the noisy frame below it.

Also, since you're new to FCP, here are two free tutorials that will save you from hours and hours of head scratching and confusion. The second one is a little older so the interface looks a little different but all the concepts are solid.

Steve at Ripple Training, 60 minute FCP tutorial overview https://www.youtube.com/watch?v=HqUP7Zgeuck

Izzy Video tutorials https://youtu.be/_jvyrnQpR1E?list=PLp5SAgbM8S87n1qvv34-PAU2zMHLlXyfb

Highlight the clip and Control-click (right-click) it.

From the contextual menu select Create Storyline or alternatively press ⌘ and G simultaneously.