This mission begins automatically after escaping the police in the previous mission, or in Parker Square if you quit the mission and pick it up later. Seems like Clara has gotten herself into trouble and you'll need to tie up a few loose ends.
Make your way to St. Joseph Cemetery and once you're kicked out of the car head toward the waypoint on foot to try and reach Clara. So Clara was the one leaving the flowers on Lena's grave all this time; part of her penance maybe.
Unfortunately, you won't make it in time to prevent the hit men from shooting Clara, but you can get some well earned revenge. Equip your strongest machine gun and go to town on the nearest men and then work your way back, killing all nine enemies.
Once it is clear you can approach Clara to complete the mission.
Complete Act 4.