How does Emerson support his claim that “to be great is to be misunderstood?”

Part of where Emerson's claim is substantiated is in his
basic idea that individuals cannot appreciate what is great and valid if they blindly
follow prevailing social trends.  Simply put, Emerson believes that the truly
exceptional artist or thinker is one who is able to demonstrate something new and so
intensely brilliant that the conformist social order would fail to appreciate it because
of its radical nature.  For example, this can be seen in art.  Emerson believed that the
truly talented artist will have to accept, to an extent, that they will not be fully
embraced by the social order in which they write.  That is because Emerson believed that
great art is composed by individuals who are willing to take risks and challenge
prevailing attitudes.  Emerson believed this in his own setting and in understanding his
contemporaries.  One of the basic ideas of Transcendentalism involves being true to
one's voice and one's own identity as an artist. Should this fly in the face of social
acceptance, Emerson is fine with such a trade- off.  To this end, Emerson's philosophy
asserts his own consolidaion of intellectual merit. Simply put, Emerson was able to to
dismiss criticism of his ideas as proof that "to be great is to be misunderstood." 
Certainly, this was not his intent, but it turned out to be quite a convenient shield
from his detractors.