Speaker is online communicator (7)
I believe the answer is:
tweeter
This clue is a double definition.
'speaker' is the first definition.
(tweeter is a type of loud speaker)
'online communicator' is the second definition.
'is' acts as a link.
(Other definitions for tweeter that I've seen before include "Person sending a messages of fewer than 140 characters" , "part of speaker" , "One relaying notes" , "Robin, for one" , "piece of hi-fi" .)