clothes in poser come in two types, conforming and dynamic. Conforming clothing is in the figure category in the library, dynamic is in the props category. If you select an item for the figure you are using, eg a LaFemme item if you are using LaFemme from the character menu it will conform as you expect.

For dynamic clothing you run a simulation

These tutorials

https://www.renderosity.com/mod/tutorial_premium/?section_id=1&page_number=2

show you how