Crossword Genius

Buried shellfish is perhaps Tarka's worst nightmare? (5,5)

Ross

I believe the answer is:

otter shell

I'm a little stuck... Click here to teach me more about this clue!

'buried shellfish is perhaps tarka's worst' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.

'nightmare?' is the wordplay.
I cannot really understand how this works, but
'nightmare?' could be 'hell' and 'hell' is found within the answer.
The remaining letters 'otters' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in The Times.
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