Possible Clues:

• Krypton, for one
• Lead, for example, English people interrupting service that gets cancelled
• Earth, wind or fire, to an early scientist
• 21-Down, e.g.
• See 39 Across
• Copper, say, sorting out melee by conservationists
• Ingredient — small amount
• Periodic-table component
• One may be glowing if one is in one's this
• Not all Matabele men take part