Speaker pretty terse before start of session (7)
I believe the answer is:
tweeter
'speaker' is the definition.
(I know that Tweeter is a type of speaker)
'pretty terse before start of session' is the wordplay.
'pretty terse' becomes 'twee' (I can't justify this - if you can you should believe this answer much more).
'before' is a charade indicator (letters next to each other).
'start of' means to remove the last letter.
'session' becomes 'term'.
'term' with its final letter taken away is 'ter'.
'twee'+'ter'='TWEETER'
(Other definitions for tweeter that I've seen before include "one sending message?" , "Chirpy type" , "Person sending a messages of fewer than 140 characters" , "one delivering a short message?" , "Message sender" .)