Is there a substitute for tahini in hummus?
Sort of.

If you have sesame seeds on hand, grind some up. You could also use toasted sesame oil, and even combine it with the ground seeds.

You can also use all-natural peanut butter. Don't use mass market crap with sugar and other additives. The ingredients should list only: peanuts, salt. Obviously, this will taste like peanuts. It will still taste good in a hummus, but it will be a distinctly different hummus than with tahini.