If an effect refers to a Clue, it means any Clue artifact, not just a Clue artifact token. For example, you can sacrifice Wrench to pay for Alquist Proft, Master Sleuth's activated ability.

Clue is an artifact type. Even though it appears on some cards with other permanent types, it's never a creature type, a land type, or anything but an artifact type.

A creature has been dealt excess damage if one or more sources dealt more damage to it than the minimum amount of damage required to be lethal damage. In most cases, this means damage greater than its toughness, but consider the damage already dealt to it this turn.

Some spells and abilities that investigate may require targets. If each target chosen is an illegal target as that spell or ability tries to resolve, it won't resolve. You won't create any Clue tokens.

You can't sacrifice a Clue to pay multiple costs. For example, you can't sacrifice a Clue token to activate its own ability and also to activate Alquist Proft, Master Sleuth's ability.

Some abilities trigger "whenever you sacrifice a Clue". Those abilities trigger whenever you sacrifice a Clue for any reason, not just to activate a Clue's activated ability.

Even 1 damage dealt to a creature from a source with deathtouch is considered lethal damage. If Torch the Witness has deathtouch (perhaps due to the effect of Judith, Carnage Connoisseur), any amount greater than 1 damage will cause excess damage to be dealt, even if the total amount of damage dealt to that creature this turn isn't greater than the creature's toughness.