Doctor Clendenning was a scientist and resident of the Omicron Theta colony before its destruction. His research involved new methods of detecting gamma radiation. He was one of many colonists whose experiences and memories were partially implanted into Data.
Clendenning was killed by the Crystalline Entity when it attacked the colony in 2336. However, records of his research remained available to other Federation scientists and his knowledge within Data. In 2368, Clendenning's unpublished theories relating to gamma radiation allowed Data and Dr. Kila Marr to determine that the Crystalline Entity left antiprotons in its wake, providing a way to track the Entity. (TNG: "Silicon Avatar")