Put it around her neck and demand answer

You grab her and put the knife around her neck, "I win. Now honor your commitment and tell me where is your clan and what is it you want?"

"Fine. My clan is at some a few miles north of this City. They do not know the existence of this City and I was on a mission to scout the city so we could raid it."

"What is the strength of the clan?"

"We have about 20 soldiers heavily armed with weapons we looted from some other cities,"

"Thank you! I hope for your sake this information is accurate," you say as you order your guards back in and order them to keep the barbarian in captive.

"Put our resources towards developing granaries and houses for our people," you order the minister. You look at the commander, "get your men ready, we are attacking the clan in the night. They have more men, but barbarians are known to be **** in the night."