When faced with a Fermi question in an interview and you have no clue about a certain assumption, follow these steps:

Acknowledge the Uncertainty: Start by acknowledging that you are unsure about the assumption. This shows honesty and transparency.
Example: "I'm not entirely sure about the exact value for this assumption, but I will make an educated guess."
Use General Knowledge: Draw on any general knowledge you might have related to the topic to make a reasonable estimate.
Example: "I know that the population of a large city can range from 1 to 10 million people, so I'll use 5 million as a middle-ground estimate."
Explain Your Reasoning: Clearly explain the logic behind your guess. This demonstrates your problem-solving approach and how you handle uncertainty.
Example: "Given that many major cities have populations in the range of a few million, I'll assume 5 million for simplicity, which seems reasonable for a large metropolitan area."
Seek Feedback: If possible, ask the interviewer if the assumption seems reasonable. This can provide guidance and show that you are open to input.
Example: "Does that assumption sound reasonable, or would you suggest a different approach?"
Proceed with the Estimate: Use your assumption to continue solving the problem. Ensure that your subsequent calculations are logical and consistent with the assumption made.
Example: "Based on the assumption of 5 million people, I'll calculate the proportion who might use a certain service..."

By following these steps, you demonstrate your ability to handle uncertainty effectively, think critically, and communicate clearly, all of which are valuable skills in any problem-solving scenario.