Possible Clues:

• Unbreakable
• Convict gets outside a prison? It's very hard
• Press subject to players - that's incontestable!
• Certain to have made a fuss, trapped by trick
• Inflexible, as some rules
• The sort of alibi which has not been forged
• Kind of stomach
• Rigid
• Inflexible, as an agreement
• Rigid group of players meeting press