It can let actors down on stage (4,4)
I believe the answer is:
trap door
This could be a double definition.
'it can let actors down' is the first definition.
The definition suggests a singular noun which matches the answer.
'stage' is the second definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'on' is the link.
Can you help me to learn more?
(Other definitions for trap door that I've seen before include "stage exit" , "Hinged opening in floor or ceiling" , "Way through a floor" , "Hinged exit in floor or ceiling" , "opening between floors" .)