The term "gifted amateur"

"The Avengers" was a BBC television series of the 1960s, starring, most famously, Diana Rigg and (throughout) Patrick MacNee. The two played elegantly effective secret agents: MacNee's character, John Steed, a professional in service to the Crown; Riggs', Emma Peel, a "gifted amateur" (fortunately, she had her own money).

The introduction to the early shows: Extraordinary crimes against the people and the State have to be avenged by agents extraordinary. Two such people are John Steed, top professional, and his partner Emma Peel, talented amateur -- otherwise known as the Avengers."