Possible Clues:

• Terribly angry about Labour leader, get knotted!
• Really cool
• Knotty (wood)
• (Of a tree) covered with knots
• Bent and twisted, as a tree
• "Cool, dude!"
• 'Cool, dude!'
• Challenging, informally
• "Cool" in '80s slang
• Any girl, sadly, won't want one bent like an old man