A hard case (5)
I believe the answer is:
shell
'case' is the definition.
(shell is a kind of case)
'a hard' is the wordplay.
I cannot quite see how this works, but
'hard' could be 'h' (abbreviation used in pencil classifications) and 'h' is found within the answer.
The remaining letters 'sell' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for shell that I've seen before include "ammunition" , "Mollusc's protection" , "pepper?" , "Mollusc's casing" , "Empty case" .)