Getting lean means losing fat and this almost always means weight loss. It is possible for some people to reveal their pack of abdominal muscles without losing weight just by changing their body composition, however, this is likely only possible for already lean individuals who do not have a lot of belly fat to lose. Additionally, recomping can be a really slow and challenging process.
Fat is a form of stored energy. When you eat a surplus of calories, your body stores this energy for use later - typically as fat. And when food calories aren’t readily available for fuel, such as in between meals, while sleeping, working out, or any time during fasting, you utilize some stored energy to support your needs.
When you burn more calories than you consume (calorie deficit), this results in a negative fat balance and ultimately fat loss. And when you eat more than you burn, this results in weight gain. Thus, focusing on total fat loss or weight loss is required to reveal your abdominal muscles.
So in order to get lean, you need to:
Reduce your total daily calorie intake from foods and beverages
Increase your calorie burn through movement or exercise
Stay consistent