The clear command erases the current screen or output device as specified by set terminal and set output. This usually generates a formfeed on hardcopy devices.
For some terminals clear erases only the portion of the plotting surface defined by set size, so for these it can be used in conjunction with set multiplot to create an inset.
set origin 0.5,0.5
set size 0.4,0.4
Please see set multiplot, set size, and set origin for details.