Possible Clues:

• Demolition derby entrant
• Striker in school for swats wanting head dismissed
• Powerful tool for compaction
• Demolition tool
• Bumper car, often
• Rear-ending car, e.g
• Bumper car, at times
• That'll make it firm in a more than sheepish sort of way, perhaps
• It makes one even more sheepish, it seems, to have made one tight