___ Stone (British Museum attraction) (7)
I believe the answer is:
rosetta
(Other definitions for rosetta that I've seen before include "Inscribed basalt slab (found 1799), key to deciphering Egyptian hieroglyphs" , "Stone providing key to deciphering Egyptian texts" , "Famous stone's from here" , "The -- Stone is a famous British Museum exhibit" , "Source of revelatory stone" .)