Relic (6,5)
'relic' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for fossil that I've seen before include "Ammonite, for example" , "amber, say" , "Prehistoric relic" , "Ancient remain preserved in rock" , "Antiquated person" .)
(Another definition for museum piece that I've seen is " such fine work could be on display".)