Sandwich at the centre of insane turnip diet (6)

I believe the answer is:

sarnie

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'sandwich' is the definition.
(I've seen this before)

I don't understand the rest of the clue.

(Other definitions for sarnie that I've seen before include "Sandwich (slang)" , "Sandwich (informal)" , "Popular bread-based snack (colloq)" , "Familiar form of snack" , "Bread and filling snack (informal)" .)

I've seen this clue in The Irish Times.

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