Shelled reptile (8)
'shelled reptile' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for tortoise that I've seen before include "Reptile with a shell" , "Slow-moving reptile that won race against a hare" , "Slow-moving land reptile" , "he's slow to eat lettuce etc" , "Shelled creature" .)
(Other definitions for terrapin that I've seen before include "Web-footed turtle" , "Swimmer" , "Aquatic tortoise" , "freshwater creature" , "A reptile; a printer (anag.)" .)