Possible Clues:

• It's supposed to be a blend of potassium and nitrogen
• Papal pronouncement taken for granted?
• It may be presumed that that's an undertaking
• 'Let old -- yield the prize' (Dryden, Alexander's Feast)
• Summer feast that's taken for granted
• Church festival is taken for granted
• Catholic feast that's taken for granted
• Undertaking reform must pain so
• It's supposed fool can acquire nous, though not at first