Possibly hot stuff a rural dean expressed is essential (4)

I believe the answer is:

must

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'is essential' is the definition.
(similar in meaning)

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

(Other definitions for must that I've seen before include "Prerequisite" , "The mould is compelled" , "Unfermented wine" , "The imperative for mould" , "Is obliged to" .)

I've seen this clue in The Times and The Sunday Times.

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