Informal exclamation in a business letter (3)

I believe the answer is:

cor

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'informal exclamation' is the definition.
The definition suggests a singular noun which matches the answer.

'a business letter' is the wordplay.
'a business' becomes 'co'.
'letter' becomes 'r' ().
'co'+'r'='COR'

'in' is the link.

Can you help me to learn more?

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

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