Formal:
* Apparel department
* Fashion department
* Textiles department
* Haberdashery (for men's clothing)
* Ladies' wear department
* Men's wear department
Informal:
* Clothes section
* Clothing aisle
* Dress shop
* Outfitters
* Boutique (for a smaller, more specialized shop)
More specific:
* Children's wear department
* Plus-size department
* Activewear department
* Formal wear department
The best word to use will depend on the specific situation. For example, if you're writing a formal report, you might use "apparel department." If you're talking to a friend, you might say "clothes section."