Would this slim my belly?

Be careful of the salt content. Salt contributes to water retention.
I hope you meant paunch.

You’re only discussing lunch… what about the other meals of the day? You need to take all meals into consideration that is appropriate for your health condition and daily activity. Consult a nutritionist or dietitian if you really need to seriously undertake a weight loss program.

A watery soup is unlikely to offer you sufficient calories. As I recall you are a teenager and so you are probably still growing. Just moderate your consumption of bread, potatoes and pasta and other vegies for a while. Go for a walk each day. Eat small, healthy meals regularly. If you are still growing in a few weeks/months it’s likely your body will have slimmed down again naturally if you are eating healthy food and getting sufficient exercise.

Everything counts. It really depends on how many less calories you are eating per day.
If you used to have a 1000 calorie lunch and now you are having a 400 calorie lunch and you aren’t adding more caliries somewhere else it should help.

Most soups have a very high sodium content. Very salty foods retain water and lead to bloating.

It’s impossible to target a specific body part losing weight.

@mattbrowne is correct. In order to lose weight in one body part you must lower your total body fat.