1. You have to hide faces yourself before the export (ether in Mesh Edit mode, or running Auto Hide Mesh in Cloth Layer Settings).
2. Inspect outfit again and make sure all faces you do not need to be exported are hidden.
3. Check Delete Hidden Faces box on export prompt (scroll a bit to reveal that check box).
There is bug however. It might not affect hidden mesh, but might cause some other problems.
The faces are hidden alright after the export, but vertices are not. Kind of sloppy to leave loose vertices with exported model.
Better be fixed.