Speaker or messenger (7)
I believe the answer is:
tweeter
I believe this is a double definition.
'speaker' is the first definition.
(tweeter is a kind of speaker)
'messenger' is the second definition.
(a tweet is a type of message)
'or' acts as a link.
(Other definitions for tweeter that I've seen before include "one delivering a short message?" , "Poster of short message - little bird?" , "Message sender" , "highly effective in reproducing sound" , "with high frequency?" .)