Just my experience as a bug smasher pilot but I believe that in the UK an ATC 'clear to land' instruction is given only when the runway is actually clear for landing. In the USA an ATC 'clear to land' or 'clear to land number 2 (or 3 etc)' is given when there are other aircraft on the runway or even still airborne. If the aircraft in front of you is slow to vacate the runway then ATC will issue a 'go around' instruction to ensure separation. Quite routine really!