Ninja in a shell (6)
I believe the answer is:
turtle
'ninja in a shell' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for turtle that I've seen before include "Neckline and reptile" , "Swimming reptile" , "Large marine reptile (with a fashionable neck!)" , "Kind of sea-tortoise, noted for soup" , "Sea swimmer" .)