Regarded as part of the play, say (4)
'regarded' is the definition.
'part of the play say' is the wordplay.
'part of the play' becomes 'scene' ('scene' is part of 'play').
'say' shows a homophone (sound like).
'scene' is a homophone of 'SEEN'.
'as' acts as a link.
(Other definitions for seen that I've seen before include "Sighted", "Viewed", "Distinguished", "Observed or percieved", "watched".)