It's just a part of the act (5)
I believe the answer is:
scene
'part of the act' is the definition.
('scene' is part of 'act')
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for scene that I've seen before include "Vista or stage setting" , "Picture" , "Display of emotion" , "Subdivision of act in play" , "Aspect" .)