Clear Speed builds are the exceptional good at killing monsters quickly and finishing a zone or map fast. Usually, builds that are especially good at clearing may lack single target damage or sometimes even struggle to kill bosses.
Good Clear-Speed builds can be defined by the class or Ascendancy, for example, the Pathfinder Ascendancy from the Ranger has one of the fasted builds in game by using Kinetic Blast and Barrage. Other classes and ascendancies have good clear speeds also but are not as dominant as the Ranger class.
Most builds have a good clear speed once you spend some currency on them or are just good mapping builds. Builds with the Clear Speed tag exile at just that, clearing your zone or map fast.