- stone, artefact that was the key to deciphering Egyptian hieroglyphics (7)
I believe the answer is:
rosetta
(Other definitions for rosetta that I've seen before include "Famous stone's from here" , "Whence Champollion's stone" , "Inscribed basalt slab (found 1799), key to deciphering Egyptian hieroglyphs" , "Source of revelatory stone" , "Town in Egypt where famous stone was found" .)