Part of play understood when listening (5)
I believe the answer is:
scene
'part of play' is the definition.
(scene in a play)
'understood when listening' is the wordplay.
'understood' becomes 'seen' (I've seen this before).
'when listening' shows a homophone (sound like).
'seen' is a homophone of 'SCENE'.
(Other definitions for scene that I've seen before include "Section of a stage work" , "Subdivision of a play/film" , "Spectacle" , "A row - or a view" , "View -part of play" .)