Clear Rune

Effects

Copies the effects of Isaac's held rune or soul stone, similar to the Blank Card and Placebo.
Drops one random rune the first time it's picked up.

Book of Virtues: Spawns purple-glowing rune wisps on the middle ring. The wisps spawn Runes/Soul Stones when destroyed, and enemies killed by the wisps' tears have a 15% chance to drop a Rune/Soul Stone. The effect of the wisps and the types of runes they spawn are independent from Clear Rune's charge time and the most recently used rune.
(Butter!) Butter!: Because Clear Rune is dropped upon usage, the recharge time for them is reset to 4, allowing to use more expensive runes much more frequently.
Car Battery: Essentially uses the rune or soul stone twice.
(Rune of Jera) Rune of Jera: Allows Isaac to copy pickups with every use. This combination can double Lil' Batteries, which can both circumvent the rune's high charge time and allow for game-breaking combos with repeated use of the synergy.

Tarot Cloth: No effect.
Void: If Void absorbs both Clear Rune and a consumable-producing item like Mom's Bottle of Pills the latter takes effect first, shifting any rune you are holding out of the primary consumable slot and rendering the Clear Rune effect useless.