Possible Clues:

• Darkness resulting from cuts in essentially indispensable energy
• Overshadow some movie clips eventually
• Upstage, say
• See 30-Down
• Programming tool whose logo features a crescent moon
• Block out tabs containing content of YouTube
• Sun-blocking astronomical event
• Section of hose clip secures sunshade?
• Shadow-creating event
• Overshadow cuts within Europe's borders