Clairvoyance allows you to map an area of the dungeon around you, showing corridor walls and stairs. This is similar to magic mapping but somewhat limited. Where magic mapping reveals the whole level, if it can be mapped, Clairvoyance will only reveal a 19×11 region around you. Like magic mapping, the map will remain unlit and will not reveal secret doors, traps or objects. Importantly however, Clairvoyance will reveal the map surrounding you on levels that will not be mapped by magic mapping, such as the Valley of the Dead.
If you have Clairvoyance as an intrinsic or extrinsic, the effect of mapping the area around you is not constant but will occur with 50% probability every 15th turn. Due to a bug, you can trigger that 50% chance over and over by walking into a wall (or with non-time-consuming commands like ; if there is no wall handy) on turns divisible by 15 until you eventually get your surroundings mapped.
You can only ever get intrinsic Clairvoyance temporarily, for a random length of between 500 and 999 turns, by donating between xplevel*200 and xplevel*400 to an aligned priest.
In SLASH'EM you can also get a single application of clairvoyance by drinking an uncursed Potion of Clairvoyance.