Prime Minister lost plot (4)

I believe the answer is:

eden

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'prime minister' is the definition.
(Anthony Eden, UK PM 1955-57)

I cannot understand how the rest of the clue works.

(Other definitions for eden that I've seen before include "Adam and Eve's garden" , "Tory Prime Minister, 1955-57" , "Place of complete bliss" , "PM, 1955-57" , "Someone once seen in Number Ten's" .)

I've seen this clue in The Independent.

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