Word for a part of a city that is under the control of organized crime?

Solution 1:

While gangland is most often used as an adjective or an attributive noun, as in, a gangland killing, it can be used to describe a specific territory controlled by gangs. Per the Urban Dictionary:

gangland
Territory owned and controlled by major street gangs.
“Don't be stepping up into that 'zoo', that's major gangland.”

See also https://en.wiktionary.org/wiki/Citations:gangland#English

• ganglands: soc.culture.african.american - 19 Apr 1992 by Stanford Guillory
  She has garnered national acclaim because her neighborhood is in the center of the ganglands of LA.

While this use is rarer, it is attested. If you were to write “Timmy went down to gangland,” a typical reader would understand that you meant that literally, that Timmy went into the turf of some gang, rather than just figuratively.