Advocate was a title in Klingon society granted to a lawyer who was tasked with defending the accused at trial.
In 2152, Kolos served as Jonathan Archer's advocate when Archer was put on trial at Narendra III. (ENT episode: "Judgment") Archer would continue to call Kolos advocate years after the incident. (ENT - The Romulan War novel: To Brave the Storm)
In 2293, Worf served as James Kirk's and Leonard McCoy's advocate when they were put on trial for the assassination of Gorkon. (TOS movie: Star Trek VI: The Undiscovered Country)
In 2369, Benjamin Sisko offered to provide an advocate for Croden for his trial for murdering Ah-Kel. (DS9 episode: "Vortex")