Enid is upset at not being out (6)

I believe the answer is:

inside

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'not being out' is the definition.
I can't judge whether this definition defines the answer.

'enid is upset' is the wordplay.
'upset' indicates anagramming the letters.
'enid'+'is'='enidis'
'enidis' is an anagram of 'INSIDE'.

'at' is the link.

Can you help me to learn more?

(Other definitions for inside that I've seen before include "Behind bars (sl.)" , "close to centre" , "Within - interior" , "Internal" , "banged up" .)

I've seen this clue in the Evening Standard.

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