What is synaptic clearance?
"Synaptic clearance" is referring to the clearing of a neurotransmitter from a synaptic cleft.  A synapse is a place where one neuron can stimulate another neuron. The tiny gap between the neurons is called the synaptic cleft. The presynaptic (stimulating) neuron releases a neurotransmitter (such as dopamine) into the cleft and some of the neurotransmitter is recognized by receptors in the membrane of the postsynaptic (stimulated) neuron.  In order for the synapse to stimulate the postsynaptic neuron a second time, the concentration of the neurotransmitter in the cleft must be reduced to its original levels.  This reduction is called clearance, and can occur in several ways (mainly dependent on the type of neurotransmitter).  The neurotransmitter may be transported back into the presynaptic neuron for reuse; this is called reuptake.  The neurotransmitter may also be degraded (broken down chemically) into an inactive form.  To a lesser extent, neurotransmitter can also be absorbed by astrocytes or simply diffuse away out of the cleft.