Strip -- no, seriously

"I'll do anything you want," said Clara.

"Anything?"

"Yes, anything."

"Okay. Take off your clothes."

"What?" said Clara.

"Take off your clothes," said Missy.

"Why should I do that?"

Missy sighed. "Because to rescue your friend the Doctor, we're going to have to sneak into the Dalek city. And the only way to do that is to go through the sewers. You don't want that beautiful outfit of yours to get ruined, do you?"

"No," said Clara.

"Then you're going to have to take those clothes off so that they won't get wet. After all, Dalek sewer water can be very smelly -- and very hard to get out of clothing."

"Then how come you're not taking off your clothes?"

"Because my clothes are made of special material, especially resistant to Dalek sewer water. However, your clothes -- being from that place you call Earth -- are not."

"Very well, " said Clara. "But do I have to take off my clothes here?"

"Why not? The sooner you do it, the sooner we can go rescue the Doctor."