This can be earned in Mission 2, Piccadilly.
For Circus Tour, there are 4 spots where you need to kill an enemy from. Once you see the street covered in fire, go left into the street. Move up a bit and you'll get to an area with a lot of terrorists. To the right of the guy standing on a car shooting down, you'll find the Aural Chic with 1 enemy inside. When facing the Aural Chic, turn left to find one of the 2 underground entrances. After entering, move to the other side and a suicide bomber should rush in. Shoot him in the head. Leave this subway where you entered and go almost straight ahead (across the street from Aural Chic). The other Subway entrance is there, with 2 enemies inside. Leave this subway from the other side where you entered. Then immediately on your left is the Reading Place (bookstore), which you need to kill all hostiles in, in order to progress the mission.
Video guide by Maka91 for those interested: