Crossword Genius

Small firm has nothing to sound happy about (3)

Ross

I believe the answer is:

coo

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

'sound happy about' is the definition.
I can't tell whether this defines the answer.

'small firm has nothing' is the wordplay.
'small firm' becomes 'co' (a firm is a company. I am not sure about the 'small' bit.).
'has' is a charade indicator (letters next to each other).
'nothing' becomes 'o' (O resembles 0).
'co'+'o'='COO'

'to' acts as a link.

Can you help me to learn more?

(Other definitions for coo that I've seen before include "Call like a dove" , "Goodness me!" , "Dove's noise" , "I'm surprised" , "(Of pigeon) murmur" .)

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