Possible Clues:

• Help for referees – it includes an alternative form
• Alternative
• Unusual form of verse at front of song book
• Form; version
• Exceptionally, rain gets into tank: not normal
• Five books include operatic song in slightly different form
• Non-standard version
• Brave, changing sides? That's different from the norm
• An event that departs from expectations
• Alternate, as spelling