Unworldly Clutch is a passive Mental Skill that works automatically with The Passage. It allows Scarlett to convince spirits of the dead to part with precious worldly possessions. The presence of such restless spirits in the mortal world is usually signaled by a Skeleton lying on the ground in the mortal realm. Scarlett can can see the ghosts with the Eye of the Lost, and speak with them using The Whisper by invoking The Passage. Unworldly Clutch lasts as long as The Passage, and has no additional cost in Mental Energy beyond what's required to invoke The Passage.
A few ghosts will just wish to rid themselves of some secret item, by giving it to Scarlett, before they move on to Death. Only a few of those items will be interesting (anything from a beer, a Treasure Map or occasionally a vital quest item. This skill is important in fulfilling certain quests (e.g. Lost Love, Glory Forsaken, and several of the guild-assigned quests if Scarlett joins the Net of the Mask.)
Unworldly Clutch is a granted skill, bestowed on Scarlett by Benedict's ghost at the Gateway Idol in the Old Cathedral of the Inner City (which she will eventually enter as part of the On Holy Mission quest, if not earlier.) Learning it doesn't require any Skill Points. It is the initial skill of the Forth Twilight Circle (even though it is probably the third Circle Scarlett opens.) Obtaining it opens the rest of the forth circle for normal learning through Necromancy Teachers and Skill Points.