Cephalopods (9)
I believe the answer is:
octopuses
'cephalopods' is the definition.
(octopus is a kind of cephalopod)
This is the entire clue.
(Other definitions for octopuses that I've seen before include "With eight long tentacles, they live in the deep" , "Multi-armed sea creatures" , "Sea-creatures with eight tentacles" , "Sea molluscs with eight arms" , "Eight-armed cephalopods" .)