Possible Clues:

• Joke used repeatedly ... and aptly found in 19-, 26-, 38-, 46- and 54-Across
• With its repetition laughter should come easy -- then silence
• Calvin's increasingly outrageous snow art, in "Calvin and Hobbes," e.g
• Repeated joke
• Recurring joke
• On potty, almost constant source of amusement
• Joke that gets funnier with repetition
• Working to suppress crack that keeps appearing