Prehistoric reptile (8)
I believe the answer is:
dinosaur
'prehistoric reptile' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for dinosaur that I've seen before include "relic" , "Extinct beast" , "type unable to adapt" , "Prehistoric monster found on Darius" , "One as T. Rex, triceratops" .)