Unpleasantly damp feeling of tunnels with shellfish about (10)

I believe the answer is:

clamminess

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

'unpleasantly damp' is the definition.
'clamminess' can be an answer for 'damp' (thesaurus). I'm not certain of the 'unpleasantly' bit.

'feeling of tunnels with shellfish about' is the wordplay.
I cannot really see how this works, but
'shellfish' could be 'clam' (clam is a kind of shellfish) and 'clam' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

I've seen this clue in The Guardian.

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