Possible Clues:

• One's stable, though angriest when unstable
• Angriest outpouring from one at the edge of a table
• Unreactive 'noble' element, such as argon
• Krypton, e.g.
• Xenon or neon
• ANAGRAM #2 (with 47-across)
• It's argon? Neon? Neither taking on being reactive
• One expects no reaction from such awful ingrates
• Passive element in the helium group
• Neon sign at heart of eery flickering