Possible Clues:

• Gag Noel performing lasting a considerable time
• Silver rings solidify on evergreen
• A set performing, good over a period of many years
• Like the wait for a 20-Across championship
• Having existed for many a year
• Unending
• 'Where the Heavenly Hours/ Weave the vast circle of his ——— day' (Southey, The Curse of Kehama)
• Enduring tree's root impedes silver pine
• Really drawn-out
• A medal worn by the Spanish for an eternity?