Creating a camera flash effect or white out in FCP

Render a slug of solid white. (little drop down menu in lower right of your viewer window)

Place this onto your timeline on a track above your video.

change the controls of the slug, double click it, now inside the viewer look at the tabs at top.

set keyframes so that the white starts at 0% opacity, hits 100% fast and goes back to 0%.

you can do this in a number of ways, thats the basic idea. but you can get alot fancier.