Creature with hard top or otherwise (9)

I believe the answer is:

arthropod

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But I can't explain it at all! Could you help me understand?

(Other definitions for arthropod that I've seen before include "spider, for example" , "One has external skeleton" , "worker, perhaps" , "Creature such as insect or crustacean" , "Such as a crustacean or spider" .)

I've seen this clue in The Sun.

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