Crossword Genius

What one may say after seeing the bill? (3)

Ross

I believe the answer is:

coo

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'bill?' is the definition.
The definition and answer can be both to do with communicating as well as being verbs in their base form.
Maybe they are linked in a way I don't understand?

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for coo that I've seen before include "Sound made by a pigeon" , "Sound like a dove" , "Make loving sounds -bill and . . ." , "Sound made by dove" , "Murmur of a dove" .)

I've seen this clue in The Globe and Mail.
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