Possible Clues:

• Monastery musical piece
• "U.S.A.! U.S.A.!," e.g
• Act as vicar maybe, providing church with a set of holy books
• Something 'U.S.A.' may be part of
• Repeated rhythmic phrase
• Sound from a picket line
• Gregorian ___ (Middle Ages musical tradition)
• Mantra, often
• Feature of many a sit-in
• Informal discussion about mid-evening prayer?