This person is correct.
I’ve finished, painted, sealed, and repaired enough stuff to know that “maybe try clear nail polish” is a terrible idea for OP.
Nail polish is very hard to get to go on smoothly over a large area and OP is very likely to end up with an uneven or marred finish. If you just put down nail polish, you are not going to get a lasting finish on much of any material without cleaning and degreasing it first.
I would clean/degrease (I don’t know what to use as I can’t tell what the material is). I would lean toward using some type of spray top coat (again, I don’t know which product because I don’t know the material). You still need to know how to spray on finishes, but I find it much easier to do well than brush on finishes.
My best guess is to use a spray lacquer for enamel, such as this:
https://a.co/d/ixJ7knW
I suspect the “brass” over zinc that is on that clip isn’t just brass - if it were brass plated zinc, I’d be totally comfortable using that spray lacquer over it. Instead, it’s probably some type of brass colored paint (or paint containing actual brass.
This means that you have mystery paint (and mystery “enamel”) that you are trying to coat. Because of that, no matter what top coat you choose, there is a chance it won’t work well and the solvents in the top coat could melt/harm the clip.
But… if I were in your situation, I would try the spray above and cross my fingers.
If you go that route:
Gently dab a bit of rubbing alcohol somewhere that you won’t be too sad if it hurts the finish. Give it a little rub. If it doesn’t mess with the finish, then you’re probably good to use rubbing alcohol to do final cleaning before top coating with lacquer.
Spray a tiny bit of the lacquer on a something (like the corner of a paper towel. immediately rub it on a spot on the clip to do the same type of test. Is the finish ok? Then you’re probably good to proceed.
Before spraying, clean the clip gently but thoroughly with dish soap and water. Rinse well. Dry well.
Give the thing a good rub with a soft cloth and rubbing alcohol to get any other remnants/grease off the clip before painting.
Give the clip a light coat of spray enamel. Follow directions for ambient temperature and recoat time. Give it another coat a bit heavier. Maybe even a third.
Let it dry and cure for a long time. The lacquer will seem completely dry and “done” after a day, but it won’t be fully cured. Giving it time to fully harden before messing with it too much.
Other notes: dust is your enemy. It’ll stick to your top coat while it’s drying. Try to minimize dust and similar nasties.
If you haven’t done used spray cans much, learn to use them first. Do a practice pass on something you don’t care about. It’s really easy to put on too much and have your clearcoat run. No fun.