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Basics of sandwich providing livelihood (5,3,6)

Ross

I believe the answer is:

bread and butter

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'livelihood' is the definition.
(synonyms)

I don't understand how the rest of the clue works.

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(Other definitions for bread and butter that I've seen before include "pudding" , "sandwiches?" , "Subsistence" , "Food" , "basic source of income" .)

I've seen this clue in The Daily Express.
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