Possible Clues:

• Unbreakable
• Armour-plated – old battleship
• Loophole-free
• Civil War fighting ship
• Like an inflexible agreement
• Unbreakable, like an alibi
• Impossible to contradict element at college working with horses
• Like a rock-solid contract
• Remove wrinkles from head of cabin boy in ship
• Like agreements you can't break