The Gangs of Novigrad - Novigrad
You'll receive this side quest automatically while doing the Get Junior Story Quest in Novigrad and it is only available while you're doing that quest. Completion of Get Junior will either fail or complete this quest for you. All you have to do for this quest is go speak to Cleaver in Novigrad and tell him you're willing to help his men in each of the three areas - the same areas you visit during Get Junior.
Clearing out both the Arena and the Casino using Cleaver's men will complete this quest. Once you're done just return to Cleaver and speak with him to complete this quest.