Greek descendant's animal discovered in vast geological age (6)
I believe the answer is:
epigon
'greek descendant's' is the definition.
(the Epigoni were descendants of Greek heroes)
'animal discovered in vast geological age' is the wordplay.
'animal' becomes 'pig' (I've seen this in other clues).
'discovered in' is an insertion indicator.
'vast geological age' becomes 'eon'.
'pig' inserted inside 'eon' is 'EPIGON'.
(Other definitions for epigon that I've seen before include "son" , "Inferior follower or imitator" , "one of an undistinguished generation?" .)