Small firm has nothing to sound happy about (3)
I believe the answer is:
coo
'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" .)