I'd guess the timeframe on this one is 60's or 70's. As best I can remember, the story unfolds like this:
An alien race, which I believe is feline in nature, has been beating humanity badly in an interstellar war. Their fleet is in the solar system, and is planning their final assault on Earth.
They've captured a human solider/fleet officer, and are interrogating him on board the alien flagship. I believe it is the leader of the aliens himself doing the questioning. The alien leader is the closest thing in the story to a protagonist.
The human has been searched and scanned for weapons/explosives/etc. He is unarmed, but they did notice that he has a hollow tooth that is leaking or pumping small amounts of adrenaline into the man's system. It's assumed this is to help sustain the man's endurance, and is left alone.
A dialogue begins between the alien leader and the man. The subjects (IIRC) being demands to know the human's fleet strength, etc. The man resists the questioning until near the end.
The surprise (for the aliens) comes when the human fleet is spotted heading towards the alien armada. Command has realized they cannot defeat the aliens in normal combat, so they have come up with a novel tactic. The entire human fleet has been under constant acceleration since Earth. By the time they near the alien fleet, their vessels will have reached relativistic speeds. Meaning they will impact with such force and energy, the alien fleet will be completely annihilated. They have become, in the words of the captured human "the biggest cosmic rays in the universe."
Before the alien leader can do anything about the horrifying realization, the captured human bites down on his hollow tooth and releases all of the adrenaline at once. He bursts out of his restraints and kills the guard(s) near him. The alien leader tries to stop him, but is hit with such force by the that it shatters his rib cage.
The story closes with the alien leader pondering how this could have happened. He takes a deep breath, and realizes that was a terrible mistake, as he feels his lung being punctured by one of his broken ribs. His last thoughts are that he notes the berserking human is heading in the direction of the ship's reactor core, no doubt to destroy the alien command vessel from within. The human was, in fact, meant to be captured and to buy time for the fleet.