Part of act heard and understood (4)
I believe the answer is:
seen
'understood' is the definition.
(I've seen this before)
'part of act heard' is the wordplay.
'part of act' becomes 'scene' ('scene' is part of 'act').
'heard' indicates a 'sounds like' (homophone) clue.
'scene' sounds like 'SEEN'.
'and' acts as a link.
(Other definitions for seen that I've seen before include "Witnessed" , "Viewed" , "Sighted" , "Espied" , "Perceived by sight" .)