Gathered there's a transmitter for the audience (7)
I believe the answer is:
amassed
'gathered' is the definition.
('amass' can be a synonym of 'gather')
'there's a transmitter for the audience' is the wordplay.
'there's' is a charade indicator (letters next to each other).
'transmitter' becomes 'mast' (as in a radio mast).
'for the audience' indicates a 'sounds like' (homophone) clue.
'mast' is a homophone of 'massed'.
'a' put next to 'massed' is 'AMASSED'.
(Other definitions for amassed that I've seen before include "Accumulated, gathered" , "Got up" , "Heaped up, accumulated" , "Accumulated, piled up" , "Put together" .)