Stage is plain to the audience (4)
I believe the answer is:
step
'stage' is the definition.
(I've seen this before)
'plain to the audience' is the wordplay.
'plain' becomes 'steppe' (steppe is a kind of plain).
'to the audience' indicates a 'sounds like' (homophone) clue.
'steppe' sounds like 'STEP'.
'is' is the link.
(Other definitions for step that I've seen before include "Put foot on a pest" , "Movement in dancing" , "Measure - pace" , "Football" , "Dance move" .)