What's the best way to kill a scratch in the body's clear coating? Thank you.

depends how deep it is. if just in the clear coat, you can wet sand it with 1500-2000-3000 grit paper than polish it out. if it goes to the paint you'll have to touch it up

are you sure the scratch is in the clear coat. . ?? Paint shrinkage in the primer and base coat looks the same at first glance.

Take a tooth pic and dip in the bottle of clear, then touch the tip into the scratch, surface tension will pull the clear into the scratch. get a new tooth pic every 2 or 3 dabs. Dont try to paint in the whole scratch, one dab at a time. you may need to do a few attempts to build up the clear. You can color sand and polish out once done.

Many thanks. How do I color sand & polish out?

1500grit and 2000 grit sandpaper. You tube the process. You may be able to just hand polish and get a decent result. I would try that first if your are not familiar with color sanding.