Informal exclamation in a business letter (3)

I believe the answer is:

cor

I'm a little stuck... Click here to teach me about this clue!

Here is my best explanation:

'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' acts as a link.

Can you help me to learn more?