Possible Clues:

• Dull routine work
• Barnaby put into uninteresting and tedious work
• Tedious, hard work
• Barnaby employed in uninteresting, monotonous work
• Dickensian character, prisoner of dull toil
• Dickens' hero involved in dull and menial work
• Grip's owner employed in uninteresting menial work
• Quietly humorous about Barnaby getting the donkey-work
• Hard, tedious work
• Menial hard work