Reptile with a bony shell (6)
I believe the answer is:
turtle
'reptile with a bony shell' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for turtle that I've seen before include "See 7" , "eg Razorback" , "Sea creature like tortoise" , "Cold-blooded creature" , "Large marine reptile (with a fashionable neck!)" .)