Savings invested in Shell? (4,3)

I believe the answer is:

nest egg

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'savings' is the definition.
(nest egg can mean someone's savings)

I cannot understand how the rest of the clue works.

(Other definitions for nest egg that I've seen before include "Reserve funds" , "Sum of money put by as a reserve" , "Savings kept in reserve" , "Rainy day savings" , "Fund put by as a reserve" .)

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