Hacking missions can appear on the map or when you intervene during a mission and consist of moving in a sort of maze to reach the exit cell. Story-related hacking missions are not timed while the others are, and you can see if it's timed by the bar above the maze.
In order to unlock this achievement, you need to complete a timed mission within the first 2/3 of the allotted time. It is fairly easy to do it during the first timed hacking missions.