He's a hard case (10)
I believe the answer is:
crustacean
'hard case' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'he's a' is the wordplay.
I cannot quite understand how this works, but
'a' could be 'an' and 'an' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for crustacean that I've seen before include "Lobster perhaps" , "Hard-shelled animal" , "Perhaps crab" , "life form with hard case" , "seafood" .)