Creature with shell, or hard top, alternatively (9)
I believe the answer is:
arthropod
'creature with shell' is the definition.
'or hard top alternatively' is the wordplay.
'or hard top' can be anagrammed to 'ARTHROPOD'.
Though, I am not sure how this is indicated.
(Other definitions for arthropod that I've seen before include "spider, for example" , "Invertebrate" , "worker, perhaps" , "Creature such as insect or crustacean" , "Such as a crustacean or spider" .)