As you are going up the stairs to chase Clarence, you will see a green health kit. 
Do not take it. Continue after Clarence and follow the advice of McCleary about not
using your weapon until you are on the second floor; otherwise, Clarence will just 
get into a vehicle and leave. If you wait until you are on the second floor, he will
climb to the roof. After you have chosen the Clarence's fate, you automatically have
a three star wanted level regardless of what you selected. You must battle your way 
out. Because you are going down, you have a distinct disadvantage in cover and will 
take lots of damage. To give yourself an edge, at the start of the mission park a 
fast car on the street near the opening in the gate. Do not pull up to the gate. Use
the stairs to your right to give chase. Once you have gotten to the roof and chosen 
his fate, descend to the level with the first aid kit (green box). Because you left 
it there on your way up, you will know what level you are on and can power up. Go 
to the side of the building where your car is parked and jump over the railing. Upon
landing, you will not lose much health. Sprint to your car and get away from the 
cops. If done correctly, you should not take a single bullet from police. 
The health pack is a requirement to survive the fall.