Slow reptile (6)
I believe the answer is:
turtle
'slow reptile' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for turtle that I've seen before include "Aquatic or terrestial reptile" , "Swimming reptile" , "Reptile - (kind of) dove" , "Marine reptile of the order Chelonia" , "Mock or Teenage Ninja?" .)