Bird, one leaving a message? (7)
I believe the answer is:
tweeter
This clue is a double definition.
'bird' is the first definition.
'one leaving a message?' is the second definition.
(someone writing a tweet on Twitter)
(Other definitions for tweeter that I've seen before include "one delivering a short message?" , "part of speaker" , "one sending message?" , "little Speaker" , "piece of hi-fi" .)