the.com/ringer

means A person or thing substituted for another, especially someone secretly entered into a competition because they're far better than the level implies.

from From 19th-century horse-racing slang, where a 'ringer' was a horse illegally swapped in for another of similar appearance to cheat the odds — likely tied to 'ring the changes,' an old phrase for substituting one thing for another. The related sense 'dead ringer,' meaning an exact look-alike, grew from the same stock: something (or someone) rung in to pass as the real thing.