To get this Clue, you must unlock the Stolen Clue Card Combo.
History
Janus Walter Raleigh stole a Clue in the Tower of London and sent it to the New World. The Lucians eventually found out and vanquished the colony, which is why it disappeared. They arrested Raleigh and put back the Clue.
Cutscene
The cutscene shows the Tower of London. Inside there is a brick with the Lucian Crest on it. The crest falls off, revealing a compartment containing a bowl of salt. The word 'SALT' will appear.
Online
The Clue Salt is unlockable by completing the "Stolen Clue" combo:
Key
They key that identifies that a card is in the "Stolen Clue" Combo contains a symbol of the pound.
Gallery
A picture of a salt shaker.