Observed part of the act we hear (4)
I believe the answer is:
seen
'observed' is the definition.
(synonyms)
'part of the act we hear' is the wordplay.
'part of the act' becomes 'scene' ('scene' is part of 'act').
'we hear' shows a homophone (sound like).
'scene' sounds like 'SEEN'.
(Other definitions for seen that I've seen before include "Espied" , "Observed or percieved" , "Distinguished" , "Met - understood" , "Perceived by sight" .)