Basically that's it: I ask for some time to think about it. This can vary from just a few moments to "I'll get back to you on this". But, of course, sometimes this isn't possible.
Not every interaction is going to be a success, you can only do your best. Creating a sense that you both see each other as human beings might give you a little more space to respond. What's most important isn't their patience, but yours. Allowing them to get their question right.