Possible Clues:

• Big-time nark
• Informer implicating many people
• If you break sugar press, he will tell on you
• Exceptional whistle-blower is great on turf
• Prolific singer likely to publicise work by The Killers?
• One of the best singers to have talked presumably about the hallowed turf of Croke Park
• Groundsman swears by it and the judge has to listen to him
• Sounds like that seed you put down in garden was brilliant for one of those particularly big rats
• Very important police informer
• 'Lovely pot' noted shopper