Large extinct reptile (8)
I believe the answer is:
dinosaur
'large extinct reptile' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for dinosaur that I've seen before include "Extinct reptilian animal" , "Large extinct creature" , "Extinct reptile - very outdated thing" , "One as T. Rex, triceratops" , "is well out of date" .)