Spoiler warning: Plot and/or ending details follow.
In 2246, Clancy was a security officer working for the colony's Security Force. During the massacre of 4,000 colonists by former Governor Adrian Kodos and his followers, Clancy was out on patrol and was not present in the New Anchorage amphitheatre when the murders took place.
Clancy was out on patrol one evening when he was cornered by a group of colonists seeking to avenge lost friends and family. He was rescued by the Starfleet Outpost survivors Lieutenant Commander Gabriel Lorca and Ensign Terri Bridges.
He then assisted Lorca's team get in to the communications center. Once inside they were able to treat injuries that both Lorca and Clancy sustained in dealing with the rioters, and Lorca was able to make contact with the USS Narbonne. (DSC novel: Drastic Measures)