Bluebird represents this silly-billy with the discontented face of regret (7)
I believe the answer is:
'bluebird represents this' is the definition.
The definition suggests a singular noun which matches the answer.
'silly-billy with the discontented face of regret' is the wordplay.
'silly billy' becomes 'twit' ().
'with' is a charade indicator (letters next to each other).
'discontented' says to hollow out the word (remove centre letters) (the word's contents are removed).
'face of' suggests taking the first letters.
'the' with its centre removed is 'te'.
The initial letter of 'regret' is 'r'.
'twit'+'te'+'r'='TWITTER'
Can you help me to learn more?
(Other definitions for twitter that I've seen before include "limited communications network" , "flutter" , "Chirrup tremulously" , "e-fad" , "networking service" .)