Sea creature with spiny shell (6)
I believe the answer is:
urchin
'sea creature with spiny shell' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for urchin that I've seen before include "Monkey" , "Roguish child - especially one raggedly dressed" , "Small grubby boy or sea creature" , "Waif" , "grubby kid" .)