A clerk was an individual whose role was primarily as an administrative aide, assisting in keep and maintaining records.
A Starfleet operations division lieutenant serving at Starbase 11 was addressed as "clerk" by Commodore Stone during Captain James Kirk's court martial trial in 2267. His duties during the trial included activating the recorder, handling identity tapes from witnesses, and later, on the bridge of the USS Enterprise, he had a tricorder, with its flap open, held over his shoulder. (TOS: "Court Martial")
In uncovering the conspiracy to infect Odo with morphogenic virus, Julian Bashir determined that at least 73 people were involved, including computer experts, doctors, security officers, admirals, and clerks. (DS9: "Extreme Measures")