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Fal: It is the more general def that I don't find. The usage to which I allude may not even be considered a separate meaning but an impromptu extension

But thanks for the suggestion, but I don't think "ringer" quite makes the grade



That's what I'm saying. The definition "something that achieves sudden, unexpected success" is the common one and is in every dictionary that I've looked it up in so far. The definition "anything ordinary on the outside but extraordinary inside" seems to be a specific usage of the definition of "ringer," "someone or something that has greater abilities than initally shown." The two terms seem to me to be potentially somewhat overlapping but I would use sleeper to refer to someone who had abilities that weren't evident because the sleeper didn't quite have it together and reserve ringer for one intentionally hiding the abilities.