A sound business letter (3)

I believe the answer is:

coo

'a sound' is the definition.
(coo is a kind of sound)

'business letter' is the wordplay.
'business' becomes 'co'.
'letter' becomes 'o' (**).
'co'+'o'='COO'

(Other definitions for coo that I've seen before include "Pigeon's sound" , "Goodness me!" , "Murmur lovingly" , "Speak lovingly -- cry softly" , "Murmur amorously" .)

I've seen this clue in the Evening Standard.

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