Possible Clues:

• The stage set for defender to fall
• Against which action is played out
• Stage curtain, e.g.
• Piece of scenery
• Setting for a scene
• Sponsor with very small amount seen in stage scene
• Painted cloth at the rear of the stage
• Stage cloth
• Reverse the end of 18 across to set the stage
• Painted setting behind stage actors, or what the ends of 17-, 21-, 33-, 42- and 54-Across can literally have