Part of act speaker's observed (5)
I believe the answer is:
scene
'part of act' is the definition.
('scene' is part of 'act')
'speaker's observed' is the wordplay.
'speaker's' indicates a 'sounds like' (homophone) clue.
'observed' becomes 'seen' ('seen' can be a synonym of 'observed').
'seen' is a homophone of 'SCENE'.
(Other definitions for scene that I've seen before include "Landscape; part of act" , "Display of emotion" , "Landscape - tantrum" , "Part of a pantomime" , "Part of Hamlet" .)