I gave a random female settler a dress because she looked better in it! Only problem is, when I trade with her, it's not in her inventory (but she's wearing it).

How do I get it back?

Put a different outfit in her inventory and make her wear it, the dress will go back to her inventory.

Thanks Rizaun!

And if you want to take ALL their stuff, put a full outfit like drifter outfit or cage armor, in the npc's inventory, make them wear it, then make them equip a small piece like a leather left leg. Everything they have will be in their inventory and they will be in their underwear, and you will be a few caps wealthier.