Hearing bums on seats … (8)
I believe the answer is:
audience
I believe this clue is a double definition.
'hearing' is the first definition.
('audience' can be a synonym of 'hearing')
'bums on seats' is the second definition.
(term for the audience in a theatre)
(Other definitions for audience that I've seen before include "Onlookers" , "Assembly gathered to watch, say, a play" , "Spectators - interview" , "Assembled spectators or listeners" , "important meeting" .)