Slow-moving reptile (8)
I believe the answer is:
tortoise
'slow-moving reptile' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for tortoise that I've seen before include "Reptile with a shell" , "Turtle-like reptile" , "One shelled" , "Slow-moving reptile that won race against a hare" , "Shelled reptile" .)