‘Clean Sweep’ is worth 10 Gamerscore and requires you to finish a Gang Attack without dying and kill at least 10 enemies in GTA Online.
The first thing that you need to do in order to unlock Gang Attacks is to reach Rank 18. After reaching Rank 18; Martin will call you and inform you about some gangs operating in San Andreas which you will need to clear out.
Taking out gang members is pretty easy to do while playing Solo but it is advised that you have a friend with you to back you up. Gang Attacks can be easily spotted by large red circles on your World Map.
Although these circles are not always visible; you will stumble upon them while free roaming a particular area. Once you enter the territory; gang members will start shooting at you at once. The number of these gang members range from somewhere under thirty.
You will need to clear the entire Gang Attack with at least ten kills at your disposal. Successfully doing this will unlock the Achievement/Trophy.
Note: You are most likely to stumble upon these gang hideouts while cruising in San Andreas or Blaine County.