Back home in the country after onset of intense exhaustion (9)

I believe the answer is:

inanition

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'exhaustion' is the definition.
(I know that inanition is a type of exhaustion)

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

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(Other definitions for inanition that I've seen before include "lack of enthusiasm" , "being exhausted" , "Exhaustion" , "Lack of vigour" , "Lassitude" .)

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

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