When is an aircraft cleared to land?
Usually, the clearance is given when the runway is clear of traffic or, at least in Europe, when two conditions are met:


Reduced Runway Separations is in use and the criteria for issuing the landing clearance are met.
The previous traffic will have left the runway when the following aircraft crosses the threshold. (Non-withheld landing clearance)



  321.5 Landing clearance need not be withheld until prescribed separation exists 
  if there is reasonable assurance that the appropriate separation will exist 
  when the aircraft crosses the runway threshold. However, do not clear an 
  aircraft to land before a preceding landing aircraft has crossed the runway 
  threshold.


Aircraft can also be made aware to expect a late landing clearance.

Clearing multiple aircraft to land in a sequence on the localizer is not used in countries operating under ICAO rules. This is more of an FAA procedure, as described in the other answers.