Deputy director was a senior position in Starfleet's Department of Internal Affairs.
Luther Sloan (of Section 31) claimed to hold this position while he posed as an agent of Internal Affairs in 2374. Sloan wore a Starfleet uniform} with a modified captain's rank insignia during his subterfuge. (DS9: "Inquisition")
The meaning of Sloan's insignia, which consisted of the four pips of a captain centered over a single gold bar, was not clear from the episode. Ronald D. Moore believed it reflected Sloan's alleged position in Internal Affairs. (AOL chat, 1998)