Remains of an ancient plant or animal (6)
I believe the answer is:
fossil
'remains of an ancient plant or animal' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for fossil that I've seen before include "Lasting impression" , "Fogeyish old thing" , "Ancient remnant hardened in rock" , "Ancient stone" , "Relic of bygone age" .)