Hard shell of some invertebrate starfish (5)
I believe the answer is:
testa
'hard shell' is the definition.
(I've seen this before)
'some invertebrate starfish' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'TESTA' is hidden in the letters of 'invertebrate starfish'.
'of' acts as a link.
(Other definitions for testa that I've seen before include "hard case" , "Hard shell" , "Protective covering" , "Seed-coat" , "cover for a seed" .)