revealer

noun

1. A person or thing that reveals.

   “Whether "The Doctor" be written by Southey or not, let revealers of mysteries determine; it is certainly written by one who strongly resembles him, perhaps too strongly to be other than himself.”

2. An entry in a crossword that guides the solver towards the theme or endgame.

   “Next (and my favorite) is Blank Space , four corner rebus squares reading "blank" for the down answers and "space" for the acrosses, mirroring the central revealers”

   “Though experienced solvers will likely see where this is going , others might need the help of the revealer”

   “The theme launches off a main revealer entry at 93-Across. There are four clues that refer to that entry, at 39-, 65-, 104- and 108-Across. When solved, 93-Across will also make the grid art snap into focus.”