Possible Clues:

• A sailor about to return to make corrections
• Mistakes made by scoundrel in a particular period
• Bookmaker's slips?
• With little hesitation, reversing a sailor's mistakes
• Mistakes are reversed - the traitor's held
• Mistakes ar made by the rodent in time
• Proofreader's headaches
• Beast in time for slips
• In time the miserable pest makes mistakes (6)
• Proof-reader's head-aches