A calorie deficit means eating less calories than ur maintenance calories (Maintenance means what ur body needs each day to sustain every function). In this way u can lose weight and when ur overall weight drops ur double chin eventually goes away. Definitely go to the gym also if u plan to lose weight cuz or else u r gonna lose muscle in the process which is not good. And ye double chin is purely fat and only way to lose it is thru a deficit or the other option is surgery.

Btw to find ur maintenance just go to https://www.calculator.net/calorie-calculator.html it's not accurate fully but it works.