Possible Clues:

• As conductor may comment on note: 'I mean it!'
• I tell you plainly up North the Derby, for one, is lacking in interest
• No argument. Spirit level shows it's OK
• End of story which is boring
• What steamroller driver might say to end argument?
• ... may provoke this criticism: there's nothing more to be said
• There's no argument which lacks sparkle
• It's rolled perfectly. No arguments!
• Informally, it's quite definite - or horizontal?