Time to remodel again and this time it's the 2nd floor. I have a space which contains the chimney flue. If during remodeling I want to enclose the metal furnce flue, what is the best way for fire safety to do this and what are the minimum clearances?

I assume this is a double walled metal chimney, correct? Manufactyrers like to specify their desired clearance from combustibles so you may be safer asking this question to your local building department. Normally 6" is safe but it is important to install proper fire stops in the cahse.