Head to the North and over to the archway just before the Hideout. There you'll see a number of Extremists and they will be attacking a number of beggars. Move in and help them by killing off the extremists. There are 4 of them that you need to deal with. If you have a Smoke Bomb, this is a great way to quickly stun then take out all of the Extremists.
They will then help Arno by giving him cover to get inside the hideout.