Ambassador sent into a bitter setting essentially (2,5)

I believe the answer is:

at heart

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'essentially' is the definition.
(I've seen this in another clue)

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

Can you help me to learn more?

(Other definitions for at heart that I've seen before include "Basically" , "In essence" , "real character" , "In one's real nature - a threat (anag)" , "Fundamentally" .)

I've seen this clue in The Irish Times.

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