Possible Clues:

• Excellent compiler with good answer
• Big, big, big: Prefix
• 6 down a splendid thing, on reflection
• Prefix with vitamins
• Prefix with ''phone'' or ''byte''
• Very great
• Grand-scale prefix
• Byte or hertz lead-in
• A jewel from the east, start of something big?
• Huge match of two halves in which there's a switch around