Invertebrates in shells (8)
I believe the answer is:
molluscs
'invertebrates in shells' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for molluscs that I've seen before include "Aquatic invertebrates" , "Invertebrates in shells" , "Cull moss to find snails, mussels, oysters etc" , "Invertebrates such as mussels and snails" , "Shellfish" .)