Mollusc providing world of opportunities? (6)

I believe the answer is:

oyster

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

'mollusc' is the definition.
(I know that oyster is a type of bivalve)

I cannot understand how the rest of the clue works.

Can you help me to learn more?

(Other definitions for oyster that I've seen before include "Pearl-making mollusc" , "aphrodisiac?" , "World as place of opportunity" , "pale pink" , "Storey (anag)" .)

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