(n.) One who advises.

Dutch uncle Polonius admonisher announcer annunciator authority backseat driver buttinsky channel cicerone communicant communicator confidant confidante consultant counsel counselor enlightener expert expert witness gossipmonger grapevine guide informant information center information medium informer instructor interviewee kibitzer meddler mentor monitor mouthpiece nestor newsmonger notifier orienter preceptist press public relations officer publisher radio reporter source spokesman teacher television teller tipster tout witness