The description for the Silver Key artifact is
Once per round, you may spend 1 less clue to pay for an effect
This boardgamegeek thread discusses it, but I am still not clear what effects this covers or doesn't cover.
My general thinking is most anything is valid as long as it isn't a specific clue that is needed to pay for an effect, i.e. a mystery that requires you to finish a research encounter and pay THAT clue toward solving it.