Angry about an anagram? (5)

I believe the answer is:

upset

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'angry' is the definition.
(thesaurus)

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

(Other definitions for upset that I've seen before include "Disconcert" , "Knock over; worried" , "Capsize" , "Distressed - surprise" , "turned over" .)

I've seen this clue in the Evening Standard.

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