Crossword Genius

An item famously sold on the coast by an unknown lady? (8)

Ross

I believe the answer is:

seashell

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

'an item' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

'famously sold on the coast by an unknown lady?' is the wordplay.
I cannot quite see how this works, but
'lady?' could be 'shell' and 'shell' is located in the answer.
The remaining letters 'sea' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

(Other definitions for seashell that I've seen before include "one picked up on shore" , "scallop?" , "Hard remains of marine mollusc" , "home by the coast, perhaps" , "Empty covering of marine mollusc" .)

I've seen this clue in The Irish Times.
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