According to freud, the part of the mind that is largely hidden and can only be brought into awareness with great difficulty, if at all, is the

Unconscious

Explanation:

Freud was the founder of psychoanalysis and he believed that most of our personality and behavior was guided based on unconscious desires and on our early childhood experiences.

According to him, the unconscious is the part of the mind that we are not aware of but that guides most of or conduct anyway, the unconscious can only be accessed through dreams, lapsus and psychoanalytical therapy.

Therefore, according to Freud,  the part of the mind that is largely hidden and can only be brought into awareness with great difficulty, if at all, is the unconscious.