I realize this may be a long shot, but I distinctly remember reading this comic, I think in a reprint anthology, probably in the early 70s. It was almost certainly a Silver Age reprint.
The gist is, nasty aliens are planning to invade Earth, but first they grab a random guy from a random city, who just happens to be Clark Kent. Unusually, Superman stays in character as Clark, and as the aliens fruitlessly subject him to various experiments and weapons, he keeps insisting, "No, I'm just average. Or a little below. Ask anybody."
Of course, the aliens decide that Earth is way too tough for them, return Clark with apologies, and skedaddle.
Has anyone got any idea who wrote or drew it, where I can find this story, (either the original or a reprint), or a way to narrow it down? Or even a clue as to where to start hunting?