Indecorous exclamation? (3)

I believe the answer is:

cor

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'exclamation?' is the definition.
Both the answer and definition are singular nouns.
Perhaps you can see an association between them that I can't see?

'indecorous' is the wordplay.
'COR' is hidden within the letters.
However, I'm not sure how this is indicated.

Can you help me to learn more?

(Other definitions for cor that I've seen before include "Blimey" , "Surprised expression" , "Expression of surprise" , "Gosh" .)

I've seen this clue in the Evening Standard.

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